PRINCIPLES' ::" 

OF 

DYNAMO-ELECTRIC  MACHINES 


AND 


PRACTICAL    DIRECTIONS    FOR   DESIGNING 
AND  CONSTRUCTING  DYNAMOS. 


WITH  AN  APPENDIX  CONTAINING  SEVERAL  ARTICLES  ON  ALLIED  SUBJECTS  AND 
A  TABLE  OF  EQUIVALENTS  OF  UNITS  OF  MEASUREMENT. 


BY 

CARL    HERING. 


NEW  YORK: 
W.  J.  JOHNSTON,  PUBLISHER, 

POTTER    BUILDING, 


' 


Copyright,  1888,  by  Carl  Her  ing. 


GLOBE  PRINTING  HOUSE, 
PHILADELPHIA. 


PREFACE. 


ENGINEERS  and  machinists  practically  engaged  in  design- 
ing, constructing  and  repairing  dynamos,  as  well  as  amateurs, 
students  of  electrical  engineering  and  electrical  artisans, 
frequently  find  it  difficult  either  to  understand  the  true 
principles  of  the  dynamo  or  to  deduce  the  proper  proportions, 
from  the  information  contained  in  existing  text-books.  This 
arises  chiefly  from  the  fact  that  much  of  the  valuable  in- 
formation given  by  physicists  unfortunately  is  not  always  in 
the  form  in  which  it  may  be  practically  applied  by  the  engi- 
neer. No  doubt  also  many  lack  that  knowledge  which  is  not 
usually  found  in  text-books,  and  which  is  obtained  only  from 
actual  experience  with  dynamos. 

To  meet  these  needs,  the  author  has  endeavored  to  give  in 
a  concise  and  simple  form  the  information  most  required  by 
those  using,  building  or  repairing  dynamos.  In  doing  so  it 
has  not  been  thought  necessary  to  preface  the  book  with  an 
elementary  treatise  on  electricity  in  general,  as  this  is  already 
contained  in  numerous  text-books;  the  author  pre-supposes, 
on  the  part  of  the  reader,  a  general  knowledge  of  the  subject 
of  electricity  and  its  applications,  that  which  is  contained  in 
this  book  being  limited  chiefly  to  details  of  the  practical  appli- 
cation of  these  principles  to  the  designing,  construction  and 
care  of  dynamos.  The  book  is  therefore  intended  more  par- 
ticularly for  builders  and  attendants  of  dynamos,  amateurs 
and  students.  It  is  needless  to  say  that  it  is  not  intended 
for  experts,  physicists  or  theorists,  nor  for  those  who  have  the 
facilities  and  time  to  search  for  such  information  in  the  more 
advanced  publications. 

Abstruse  theories  of  dynamos  have  been  entirely  omitted  as 
being  of  little  use  to  the  practical  engineer.  Until  such  theo- 


464518 


iv  Preface. 

ries  have  been  thoroughly  tested  by  repeated  and  varied 
applications  in  practice,  and  are  reduced  to  such  a  form  that 
they  may  be  readily  applied,  the  constructor  of  dynamos  is 
recommended  to  use  the  well  tried  but  less  direct  methods. 
Probably  the  best  among  the  newer  methods  of  designing 
magnets  for  dynamos,  and  one  which  appears  to  be  an  im- 
portant improvement  in  the  right  direction,  is  that  proposed  by 
Mr.  Gisbert  Kapp,  in  the  Proceedings  of  the  /Society  of  Tele- 
graphic Engineers,  November  llth,  1886,  and  subsequent 
papers,  to  which  the  more  advanced  readers  are  referred. 

It  is  assumed,  in  this  book,  that  the  reader  has  a  knowledge 
of  arithmetic  and  understands  the  application  of  simple  for- 
mulae. All  formulae,  laws  and  relations  are  given  in  as  simple 
forms  as  are  consistent  with  clearness,  but  it  is  possible  that 
in  doing  this  the  author  may  sometimes  have  sacrificed  strict 
scientific  accuracy ;  it  was  thought,  however,  that  as  the  calcu- 
lation of  the  parts  of  dynamos  does  not  admit  of  absolute  pre- 
cision, this  would  not  be  objectionable. 

The  subject  matter  of  this  volume  first  appeared  as  a  serial  in 
the  Electrician  and  Electrical  Engineer,  of  New  York,  but  it 
has  been  revised  in  numerous  parts.  The  Appendices  contain 
several  papers  by  the  author  on  allied  subjects  (Appendix  IV 
being  only  an  abstract  of  the  original),  which  were  also  pub- 
lished in  the  above  Journal,  and  which  it  is  thought  might  be 
of  use  to  the  readers  of  this  book.  To  the  table  of  equivalents 
have  been  added  those  of  work,  power  and  heat,  besides  num- 
erous others,  making  it  more  nearly  complete. 

CARL  BERING. 
University  of  Pennsylvania, 
February,  1888. 


CONTENTS. 


CHAPTER  I.— REVIEW  OF  ELECTRICAL  UNITS  AND  FUNDAMENTAL 

LAWS. 

Analogies  to  mechanical  phenomena ;  potential ;  quantity ; 
current ;  machines  generate  pressure ;  resistance ;  laws  of 
current,  work  and  power ;  analogies ;  capacity ;  ampere- 
turns;  electrical  horse-power, Page  1 

CHAPTER   II. — FUNDAMENTAL    PRINCIPLES   OF    DYNAMOS   AND 

MOTORS. 

Complicated  theories  unnecessary;  Oersted's  fundamen- 
tal experiment ;  a  motor  consists  of  a  current  and  a  magnet ; 
a  dynamo  is  a  motor  with  the  conditions  reversed ;  analogy ; 
dynamos  generate  potential ;  potential  generates  current,  .  page  14 

CHAPTER  III. — MAGNETISM  AND  ELECTROMAGNETIC  INDUCTION. 

Lines  of  force ;  magnetic  fields  expressed  and  measured 
in  lines  of  force ;  analogy  to  gravity ;  amount,  intensity, 
polarity  and  direction  of  magnetism ;  unit ;  field  around  a 
current;  rules  of  relations;  properties  of  lines  of  force; 
applications ;  laws  of  electromagnetic  induction ;  rules  of 
relations;  direction  of  current  in  a  generator;  difference 
of  potential  as  distinguished  from  electromotive  force,  .  .  page  18 

CHAPTER  IV. — GENERATION  OF  ELECTROMOTIVE  FORCE  IN  DY- 
NAMOS. 

A  wire  cutting  lines  of  force  generates  an  electromotive 
force;  four  ways  of  increasing  this  electromotive  force; 
speed;  intensity  of  field;  .size  of  magnets;  successive  cut- 
ting of  same  field ;  the  commutator  a  mere  collector ;  prin- 
ciples of  the  Gramme  and  the  cylinder  windings,  ....  page  30 

CHAPTER  V.— ARMATURES. 

Gramme  armature ;  rules  for  determining  the  polarity  of 
the  brushes ;  potential  proportional  to  the  number  of  turns ; 
counter  magnetism  of  armature;  resulting  magnetization  of 
armature;  magnetic  lag;  number  of  windings  should  be 
small;  causes  of  sparking  at  the  brushes;  commutator  in- 
sulation should  be  thin;  coils  short-circuited  by  brushes; 
neutral  field;  shifting  of  brushes  for  regulating;  self-in- 
duction ;  insulation ;  symmetry ;  magnetic  proportions  of 
armature;  iron  lugs;  Voucault  currents;  laminating  the 
cores;  effect  of  film  of  oxide  on  iron  plates, page  42 

CHAPTER  Y,  CONTINUED. — ARMATURES. 

Dead  wire  on  Gramme  armature;  flat  ring  armatures; 
(v) 


vi  Contents. 

cross-section  of  core ;  diameter  of  armature ;  speed ;  in- 
crease of  speed  a  direct  gain;  conditions  of  high  speed  ; 
armatures  balanced  statically  and  dynamically, page  56 

CHAPTER  V,  CONTINUED. — ARMATURES. 

Conductor  velocity ;  effect  of  increasing  it ;  velocities  in 
the  best  machines ;  relations  between  the  length  of  wire  and 
the  electromotive  force;  active  wire;  induction  in  volts 
per  foot ;  intensities  of  field  used  in  practice ;  cross-section 
of  wire ;  density  of  current;  depends  on  induction  per  foot ; 
current  density  in  the  best  machines;  depth  of  armature 
winding;  distance  between  pole-piece  projections;  leakage,  page  66 

CHAPTER  V,  CONTINUED.— ARMATURES. 

Winding  the  wire;  method  of  bringing  the  end  and  be- 
ginning in  the  outside  layer;  lugs;  smooth  winding;  proper 
spacing;  appliances  used  in  winding;  commutator  branch 
connections;  binding  wires;  insulation;  iron  wire  in  place 
of  copper;  laminating  the  core;  mechanical  strains  on 
armature  coils;  heating,  prevention  better  than  ventilation  ; 
commutator,  insulation  of  the  bars ;  connections  at  and  to 
the  commutator ;  brushes,  page  76 

CHAPTER  V,  CONCLUDED. — ARMATURES. 

Cylinder  armatures  compared  with  Gramme  armatures ; 
proportions  of  cylinder  armatures ;  symmetry  of  winding ; 
width  of  coils;  "heads;"  principle  of  cylinder  winding; 
details  of  winding;  different  systems;  appliances  used  in 
winding;  best  order  in  which  to  wind  the  coils;  irregu- 
larity in  Siemens'  winding ;  connecting  an  incorrectly  wound 
coil;  testing  for  correct  connections.  UNIPOLAR  ARMA- 
TURES.— Term  unipolar  applies  to  armature;  nature  of  cur- 
rents; reason  for  the  low  potentials;  Siemens  machine; 
Forbes  machine;  inoperative  high  potential  machines; 
operative  high  potential  machines.  ALTERNATING  CUR- 
RENT ARMATURES. — General  types;  advantages  over  direct 
current  machines;  curious  features  of  some  alternating  cur- 
rent machines ;  Gordon  machine  ;  objections  to  alternating 
currents ;  alternating  current  motors,  Page  87 

CHAPTER  VI. — CALCULATION  OF  ARMATURES. 

Proportions  depend  on  objects  to  be  accomplished,  and 
not  merely  on  the  output ;  trial  calculations  ;  order  of  de- 
termining different  parts;  testing  correctness  of  prelimi- 
nary calculations ;  varying  assumed  dimensions ;  allowance 
for  self-excitation;  illustration  of  this  method  by  an  ex- 
ample; importance  of  varying  the  proportions  in  the  calcu- 
lations ;  importance  of  having  the  field  intense,  ....  page  104 


Contents.  vii 

CHAPTER  VII.— FIELD  MAGNET  FRAMES. 

Proper  design  is  based  on  mechanical  as  well  as  electrical 
considerations ;  choice  of  cast  or  wrought  iron ;  quality  of 
the  iron ;  the  cores  are  the  most  intensely  magnetized ;  rel- 
ative values  of  cast  and  wrought  iron ;  size  of  magnets ; 
saturation ;  want  of  practical  and  reliable  data ;  relative 
proportions  of  different  parts ;  leakage ;  counter-magnet- 
ism of  armature ;  actual  proportions  of  field ;  proportions 
deduced  from  a  model ;  length  of  cores  ;  relations  between 
diameter  of  coils  and  cores;  calculations  of  absolute 
and  relative  intensities  of  field;  deductions  from  formulae; 
effect  of  the  iron  in  a  magnet ;  illustration  of  the  applica- 
tion of  the  formulae;  typical  forms  of  frames;  like,  parallel 
magnets  are  objectionable;  opposite  parallel  magnets  assist 
each  other;  non-magnetic  space  around  armature;  pole- 
piece  projections ;  balanced  field ;  accessory  iron  parts, .  -  page  116 

CHAPTER  VIII.— FIELD  MAGNET  COILS. 

Empirical  better  than  theoretical  determinations  of  the 
winding ;  factors  introducing  errors ;  the  determination  of 
the  winding  may  be  advantageously  left  to  the  last ;  experi- 
mental determination  of  the  ampere-turns ;  precautions  in 
making  the  test.  SEPARATELY-EXCITED  MACHINES. — Cal- 
culation of  the  winding  from  the  ampere-turns;  choice  of 
field  current;  winding  for  an  exciter  of  fixed^  potential;  for- 
mulae for  direct  calculation  of  diameter  of  wire ;  relation  of 
depth  of  winding  to  diameter  of  core ;  mean  length  of  a  turn ; 
formulae  for  same;  number  of  turns;  deductions  from  re- 
sults obtained;  guarding  against  saturation;  against  heat- 
ing; laws  and  formulae  for  heating;  the  least  diameter  of 
wire  can  be  calculated  without  knowing  the  current;  choice 
of  current;  depends  on  the  objects  of  the  designer ;  illustra- 
tion of  the  application  of  all  the  formulae  by  a  practical 
example;  relations  of  equal  volume  coils;  application ;  in- 
direct determination  of  the  winding;  deductions  from  values 
obtained;  winding  for  an  exciter  of  fixed  current;  un- 
systematic determinations;  additional  formulae  for  resist- 
ance and  potential  of  coils,  length,  number  of  layers  and 
depth  of  winding ;  determining  constant  for  heating  formulae ; 
Brough's  formulae;  modified  form;  insulation  of  coils; 
smooth  winding;  straightening  the  wire;  measuring  its 
length;  direction  of  winding  immaterial;  polarity  of  mag- 
nets; keeping  full  records.  SERIES  MACHINES.— Equiva- 
lent to  separate  excited  machine,  with  a  constant-current 
exciter;  additional  precautions;  number  of  windings  is 
fixed;  no  impossible  case;  factor  of  safety ;  the  method  de- 
scribed eliminates  many  causes  of  error ;  final  corrections  of 
speed.  SHUNT  MACHINES. — Equivalent  to  a  separate  ex- 
cited machine  with  a  constant-potential  exciter;  additional 


viii  Contents. 

precautions ;  number  of  windings  is  not  fixed ;  impossible 
case ;  factor  of  safety  ;  final  correction  of  speed.  COMPOUND 
MACHINES. — Principle  of  compound  winding ;  modification 
of  winding  for  incandescent  lighting ;  two  methods  of  mak- 
ing connections;  test  for  determining  ampere-turns;  plot- 
ting results;  determining  the  proportions  of  series  and  shunt 
coils ;  corrections  for  greater  accuracy  in  the  two  cases ;  rel- 
ative values  of  the  two  methods ;  most  desirable  propor- 
tions for  compound  machines ;  limitations  of  the  self-regula.- 
tion  of  compound  machines, page  138 

CHAPTER  IX. — REGULATION  OP  MACHINES. 

Moving  the  brushes;  objections;  proper  position  of 
brushes  not  that  for  greatest  potential ;  varying  the  speed  ; 
varying  the  external  resistance;  calculation  of  resistance 
coils  for  regulation,  for  same  diameter,  for  different  diam- 
eters, for  bands ;  varying  the  ampere-turns;  adjusting  the 
separate  exciter ;  adjustable  resistance  in  the  shunt  magnet 
circuit ;  for  constant  potential ;  for  constant  current ;  vary- 
ing the  number  of  windings  in  shunt  and  series  machines ; 
adjustable  resistance  shunting  series  coils,  for  constant  cur- 
rent, for  constant  potential ;  automatic  regulation,  .  .  .  page  187 

CHAPTER  X.— EXAMINING  MACHINES. 

Importance  of  making  an  examination  of  a  machine; 
characteristics  for  series  machines ;  deductions ;  additional 
characteristics  for  series  machines;  characteristics  in  gen- 
eral; comparison  of  characteristics;  characteristic  for  shunt 
machine;  deductions;  characteristics  for  compound  ma- 
chines; test  for  saturation;  saturation  curve;  deductions; 
details  of  saturation  test ;  application  of  data  obtained  from 
saturation  test;  importance  of  studying  characteristics; 
characteristic  for  separately  excited  machine;  deductions; 
other  tests;  counter  magnetization  of  armature ;  shifting  of 
brushes ;  exploring  the  armature  field ;  magnetic  leakage ; 
exploring  the  external  magnetic  field ;  armature  resistance ; 
heating  co-efficients ;  efficiency  of  machines, page  197 

APPENDIX  I.— PRACTICAL  DEDUCTIONS  FROM  THE  FRANK- 
LIN INSTITUTE  TESTS  or  DYNAMOS, page  215 

APPENDIX  II.— THE  SO-CALLED  "DEAD  WIRE"  ON  GRAMME 

ARMATURES, page  231 

APPENDIX  III.— EXPLORATIONS  OF  MAGNETIC  FIELDS  SUR- 
ROUNDING DYNAMOS, page  241 

APPENDIX  IV.— SYSTEMS  OF  CYLINDER- ARMATURE  WIND- 
INGS,   page  261 

APPENDIX  V. — EQUIVALENTS  OF  UNITS  OF  MEASUREMENTS 

(Table), ."  page  269 


OF 

DYNAMO-ELECTRIC  MACHINES. 


CHAPTER   I. 

Review  of  Electrical  Units  and  Fundamental  Laws. 

Electrical  phenomena,  being,  like  all  other,  resultants  of 
force  and  matter,  are  in  many  respects  quite  analogous  to 
mechanical  phenomena.  Their  nature  may  therefore,  in  many 
cases,  be  much  more  readily  understood  by  comparing  them 
with  well-known  mechanical  effects.  Understanding  the  me- 
chanical relations,  it  is  then  very  easy  to  understand  the 
electrical  ones  by  their  analogy.  The  necessary  electrical 
computations  will  then  be  much  more  readily  intelligible  and 
will  appear  quite  as  simple,  and  perhaps  even  more  simple, 
than  the  ordinary  mechanical  ones. 

Unlike  in  mechanical  phenomena,  the  quantities  in  elec- 
trical computations  are  not  measured  in  pounds,  feet,  or 
gallons,  by  reason  of  the  different  nature  of  the  forces  and 
matter ;  they  must,  therefore,  be  measured  in  units  of  their 
own.  Although  electrical  phenomena  are  not  expressed  in 
pounds,  feet  or  gallons,  it  is  quite  admissible  to  say  that  there 
are  quantities  in  electrotechnics  which  have  similar  functions 
to  distance,  weight  and  capacity  in  mechanics,  and  that  the 
analogies  to  these  may  be  used  to  illustrate  the  nature  of 
different  electrical  phenomena  and  to  assist  in  showing  how 
electrical  calculations  should  be  made. 

This  may  be  best  shown  by  an  example.  The  ordinary 
well-known  centrifugal  blower,  consisting  simply  of  a  num- 
ber of  fans  rotating  in  an  enclosed  vessel,  will,  when  in 
action,  produce  a  compression  of  the  air  at  one  part,  and 


2  Principles  of  Dynamo- Electric  Machines. 

a  rarefaction  of  air  at  another;  that  is,  it  will  force  the  air 
out  at  one  part,  and  draw  it  in  at  the  other.  If  the  two 
ends  of  a  tube  are  attached  to  these  two  points,  a  current 
of  air  will  be  caused  to  flow  through  it  in  a  definite  direc- 
tion. This  very  simple  machine  affords  a  very  good  illus- 
tration not  only  of  the  simple  quantities  in  electrotechnics 
and  the  elementary  laws  of  electrical  action,  but  also  of 
other  points  which  are  less  frequently  well  understood, 
and  by  a  clear  conception  of  which,  electric  batteries,  ma- 
chines and  phenomena  connected  with  them,  appear  much 
simpler,  and  can,  therefore,  more  readily  be  subjected  to 
quantitative  investigation. 

The  mechanical  pressure  of  air  which  is  generated  in  the 
blower,  being  of  a  positive  character  (having  a  density 
greater  than  that  of  the  atmosphere)  at  one  part,  and 
negative  (or  suction,  or  less  than  the  atmosphere)  at 
another  part,  corresponds  precisely  to  the  electrical  press- 
ure, or  potential,  in  batteries  and  machines.  In  both  cases, 
if  they  are  allowed  to  equalize  themselves,  they  will  pro- 
duce a  current  in  a  certain  direction — from  the  positive  to 
the  negative  in  the  external  circuit,  and  from  negative  to 
positive  within  the  machine  or  battery  itself.  This  press- 
ure, which  in  mechanics  is  expressed  as  so  many  pounds 
per  square  inch,  corresponds  in  electrotechnics  to  what  is 
termed  electrical  pressure,  electromotive  force,  potential,  or 
tension,  and  which  is  measured  in  volts. 

In  the  case  of  air,  the  zero  or  normal  pressure  corresponds 
with  that  of  the  atmosphere,  for  when  air  in  a  confined  ves- 
sel is  at  atmospheric  pressure,  there  will  be  no  current  pro- 
duced if  the  vessel  is  made  to  communicate  with  the  air. 
In  the  case  of  water,  the  zero  level  is  taken  as  that  of  the 
ocean.  Similarly  there  is  a  zero  level  or  pressure  of  elec- 
tricity with  which  other  pressures  can  be  compared.  'This 
is  the  natural  pressure  of  electricity  in  the  earth  itself. 
The  earth,  as  is  well  known,  may  be  regarded  as  a  reser- 
voir of  electricity  of  infinite  quantity,  and  its  pressure  is 
taken  as  zero..  If  one  pole  of  a  battery  or  other  generator 


Review  of  Electrical  Units  and  Fundamental  Laws.  3 

is  connected  with  the  earth  and  a  current  tends  to  flow 
from  it  to  the  earth,  then  that  pole  is  assumed  to  be  posi- 
tive; and  if  the  other  pole  be  similarly  connected,  the  cur- 
rent will  tend  to  flow  from  it  to  the  generator,  and  is  there- 
fore assumed  to  be  the  negative  pole.  As  almost  all 
electric  lighting  apparatus  is  isolated  from  the  earth,  it 
makes  no  practical  difference  whether  the  actual  pressure 
is  above  or  below  zero.  All  that  concerns  the  electrician 
is  to  know  what  is  the  difference  between  the  two  pressures 
at  the  two  poles  of  the  battery  or  other  generator;  hence 
the  term  "difference  of  potential."  The  zero  pressure  of 
electricity  is,  however,  important  in  considering  ground 
connections  of  machines  and  lines,  as  will  be  seen  hereafter. 

As  in  mechanics  a  pressure  is  necessary  to  produce  a 
current  of  air,  so  in  electrical  phenomena  an  electromotive 
force  is  necessary  to  produce  a  current.  A  current  in  either 
case  can  never  exist  without  a  pressure  to  produce  it. 
From  this  the  important  law  is  deduced  that  a  difference 
of  electrical  pressure  cannot  exist  at  two  points  of  a  con- 
ductor without  generating  a  current  between  these  two 
points,  except  when  there  is  a  counter  pressure  in  that  con- 
ductor in  the  opposite  direction,  as,  for  example,  in  the 
machine  itself.  This  law,  which  seems  self-evident,  is  fre- 
quently of  use  in  solving  certain  problems. 

This  current  of  electricity,  expressed  in  amperes,  repre- 
sents quantity  per  second,  just  as  it  would  do  in  an  air 
current,  and  it  is  very  similar  in  many  other  respects  to 
the  current  of  air  produced  by  a  blower. 

The  actual  quantity  of  electricity  itself  is  measured  in 
coulombs,  which  correspond  to  quantity  of  air.  Just  as  in 
the  case  of  air,  this  quantity  may  exist  in  a  state  of  rest,  as 
in  the  charge  of  a  Leyden  jar,  or  a  charged  cloud;  and  in 
that  state  it  may  be  at  any  pressure,  either  greater  or  less 
than  that  of  the  earth.  If  in  that  state  it  is  allowed  to 
equalize  itself,  through  a  conductor  to  the  earth  or  to 
another  confined  quantity  of  electricity  at  any  other  press- 
ure, a  current  will  flow  as  in  the  case  of  air.  If  this  quan- 


4  Principles  of  Dynamo- Electric  Machines. 

tity  be  limited,  as  in  a  statically  charged  Leyden  jar  or 
condenser,  it  will  equalize  itself  suddenly  in  the  form  of  a 
momentary  current  or  spark,  in  the  same  way  as  the  con- 
fined gases  of  ignited  powder  in  a  pistol  suddenly  equalize 
their  pressure  with  that  of  the  atmosphere.  If?  however, 
this  pressure,  which  is  consumed,  be  continually  replaced 
by  some  means,  in  the  same  proportion  in  which  it  is  con- 
sumed, the  current  which  was  momentary  in  the  first  case 
will  then  become  continuous.  This  is  precisely  what  is 
done  by  an  electric  machine,  battery,  or  other  generator. 
It  continually  replaces  the  electrical  pressure  which  is  con- 
sumed or  equalized  through  the  lamps  and  conductors  con- 
necting its  poles,  just  as  the  centrifugal  blower  continu- 
ally replaces  the  pressure  which  is  again  consumed  in  the 
place  where  it  equalizes  itself  or  where  the  work  is  done. 
It  will  be  seen  from  this  that  the  object  of  an  electric 
machine,  battery  or  other  generator,  is  to  maintain  a  con- 
tinuous electrical  pressure,  or  electromotive  force,  the 
amount  of  which  shall  be  equal  to  that  which  is  consumed, 
for  in  that  case  only  can  a  continuous  current  be  estab- 
lished and  maintained.  Properly  speaking,  its  object  is 
not  to  generate  electricity,  for  there  is  practically  an  infin- 
ite quantity  of  that  in  the  earth  itself,  and  were  it  only  the 
quantity  of  electricity  which  is  wanted,  there  would  be  no 
need  of  machines.  Electricity  in  quantity  without  pressure 
is  useless,  for  it  cannot  be  made  available,  under  such  con- 
ditions, for  operating  lamps,  telegraphs,  etc.  It  is  a  cur- 
rent which  is  required,  and  this  can  be  maintained  only  by 
a  constant  renewal  of  pressure,  which  is  what  the  machine, 
battery,  or  other  generator  does.  As  electricity  is  neither 
consumed  in  lamps  nor  actually  generated  by  machines,  it 
follows  that  to  each  electric  machine  or  battery  there  must 
be  two  conductors  or  wires — one  to  lead  to  it  the  supply 
of  electricity  at  low  pressure,  and  the  other  to  lead  off  the 
electricity  of  high  pressure,  while  on  the  other  hand,  in  the 
lamps  or  other  apparatus,  where  the  pressure  is  consumed, 
the  function  of  the  wires  is  precisely  the  reverse. 


Review  of  Electrical  Units  and  Fundamental  Laws.  5 

The  truth  of  the  proposition  that  electricity  is  not  actu- 
ally produced  in  the  machines  nor  consumed  in  the  lamps, 
may  be  demonstrated  by  measuring  the  actual  quantity  of 
current  flowing  in  through  one  wire  and  out  through  ano- 
ther, which  will  always  be  found  the  same.  The  action  is 
precisely  analogous  to  what  takes  place  in  the  centrifugal 
air  blower,  which,  as  we  well  know,  generates  pressure,  but 
not  air.  It  may  also  be  compared  to  a  hydraulic  pump, 
which  generates  the  pressure  of  the  water,  but  not  the 
water  itself  ;  as  much  water  must  flow  into  the  pump  at  one 
end  as  flows  out  at  the  other. 

From  these  remarks  the  following  important  laws  be- 
come self-evident  : 

As  the  pressure,  and  not  the  electricity,  is  that  which  is 
produced  and  consumed,  it  follows  that  the  current  strength 
is  always  the  same  in  every  part  of  a  given  circuit.  In 
case  the  circuit  is  divided  at  any  point,  the  sum  of  the 
divided  currents  is  always  equal  to  the  undivided  current 
in  the  rest  of  the  circuit.  The  function,  therefore,  of  an 
electric  machine  or  battery,  is  to  generate  and  maintain  an 
electromotive  force,  or  electrical  pressure.  Whenever  this 
pressure  is  allowed  to  act  or  to  equalize  itself,  a  current  of 
electricity  will  be  produced. 

The  electrical  resistance  (measured  in  ohms)  which  op- 
poses the  current,  is  also  quite  analogous  to  the  mechanical 
resistance  which  must  necessarily  be  encountered  by  every 
current  of  air.  The  only  difference  is  that  the  electrical 
resistance  depends  only  on  the  cross  section,  length,  and 
nature  of  the  material  of  the  conducting  medium,  while 
with  a  current  of  air  the  resistance  is  affected  by  certain 
other  factors,  thus  making  calculations  due  to  electrical 
resistance  much  the  more  simple  of  the  two. 

The  laws  governing  electrical  resistance  are,  that  the 
resistance  increases  with  the  length  of  the  conductor,  and 
that  it  diminishes  as  the  area  of  cross  section  increases. 
It  also  depends  specifically  upon  the  nature  of  the  mate- 
rial, or,  in  other  words,  on  a  particular  constant  for  each 


6  Principles  of  Dynamo- Electric  Machines. 

substance.  It  also  increases  with  the  temperature  in  metals 
and  decreases  in  carbon  and  liquids.  The  unit  of  resistance 
now  universally  adopted  is  the  Paris  or  legal  ohm,  which 
is  the  resistance  of  a  column  of  pure  mercury  of  one  square 
millimetre  cross  section  and  106  centimetres  in  length,  at  a 
temperature  of  0°  C.  The  Siemens  unit  is  the  same  except 
that  the  column  is  1  metre  long.  The  old  unit  formerly 
used  is  called  the  British  Association  (B.  A.)  unit.  Their 
equivalents  are  as  follows : 

1  legal  ohm        ==  1.0112  B.  A.  units.1 
1  B.  A.  unit       =    .9889  legal  ohms.1 
1  Siemens  unit  =    .9434     "          " 
1  legal  ohm       =  1.0600  Siemens  units 

The  very  simple  relation  which  exists  between  the  elec- 
trical pressure  or  electromotive  force,  the  resistance  and 
the  resulting  current,  forms  the  basis  of  almost  all  electri- 
cal computations  and  is  known  as  Ohm's  law.  This  law  is 
nothing  more  than  a  statement  of  the  fact  that  the  result  of 
any  action  increases  when  the  cause  of  action  increases,  and 
decreases  when  the  force  opposing  this  action  increases. 
As  we  have  seen,  the  electrical  pressure  or  electromotive 
force  is  the  cause  which  produces  a  current,  without  which 
a  current  cannot  possibly  exist.  We  have  also  seen  that 
the  resistance  opposes  this  current,  and  therefore  opposes 
the  action  of  the  pressure,  or  cause  of  the  current.  There- 
fore the  resultant  of  both  of  these  will  be  the 
current.  Or  stated  in  more  concise  terms,  the  resulting 
current  is  equal  to  the  electromotive  force  divided  by  the 
resistance. 

From  this  well-known  law  two  others  necessarily  follow, 
which  are  equally  useful,  though,  strange  to  say,  less  fre- 
quently used.  One  serves  to  determine  what  pressure  or 
electromotive  force  is  required  to  produce  a  certain  current 
through  a  certain  resistance.  It  follows  from  the  law  above 
given  that  this  electromotive  force  will  be  the  product  of 

1 .  As  marked  on  the  standard  legal  ohms,  by  Mr.  Glazebrooke,  secretary  of 
the  British  Association. 


Review  of  Electrical  Units  and  Fundamental  Laws.  7 

the  given  resistance  and  the  given  current.  The  other  en- 
ables us  to  find  what  resistance  is  to  be  inserted  in  order 
that  a  given  electromotive  force  shall  produce  a  certain 
current.  This  resistance  will  be  readily  seen  to  be  equal 
to  the  electromotive  force  divided  by  the  current. 

Another  very  important  law  follows  from  the  above- 
mentioned  premises.  If  a  current  divides  at  any  point  into 
two  or  more  lesser  currents,  it  follows  that  each  of  these 
divided  or  fractional  currents  will  be  greater  in  proportion 
as  the  resistance  of  its  own  circuit  is  less,  or  in  other  words, 
each  current  will  be  proportional  to  the  reciprocal  of 
its  resistance.  It  is  also  self-evident  that  the  sum  of  the 
divided  currents  will  be  equal  to  the  total  undivided  cur- 
rent. If  the  difference  of  the  electrical  pressure  or  poten- 
tial, at  the  points  where  the  current  divides  and  joins  again, 
is  known  (as  in  a  dynamo  machine),  the  calculation  becomes 
exceedingly  simple,  as  it  then  is  merely  a  double  application 
of  one  of  the  laws  above  given,  namely,  that  the  current  in 
each  branch  is  equal  to  the  difference  of  potential  divided 
by  the  resistance  of  that  branch. 

The  only  quantities,  besides  time,  which  are  of  the  same 
kind  in  both  electrotechnics  and  mechanics,  are  energy  or 
work,  and  power.  These  are,  therefore,  the  connecting 
links  between  the  two,  and  are  the  quantities  to  which  both 
must  be  reduced,  in  order  to  find  how  much  of  the  one  will 
be  equal  to  a  given  quantity  of  the  other. 

In  mechanics  it  is  well  known  that  work  is  measured  in 
foot-pounds,  while  power  (which  is  merely  an  amount  of 
work  done  in  a  certain  time  or  rate  of  doing  work)  is  meas- 
ured in  horse-powers. 

If  a  certain  quantity  of  air,  for  instance  a  cubic  foot,  at 
ordinary  pressure,  is  compressed,  say  to  half  that  quantity, 
or  what  is  the  same  thing,  to  double  that  pressure,  a  certain 
definite  number  of  foot-pounds  of  energy  will  be  required, 
and  if  it  be  allowed  to  expand  again,  that  is  to  equalize  the 
pressure  again  with  the  atmosphere,  that  same  amount  of 
work  in  foot-pounds  will  be  given  off.  It  will  be  equal  to 


8  Principles  of  Dynamo- Electric  Machines. 

the  pounds  mean  pressure  multiplied  by  the  distance  through 
which  it  acts,  in  feet. 

In  precisely  the  same  way,  when  a  certain  quantity  of  elec- 
tricity in  coulombs  has  its  electrical  pressure  increased  a 
certain  number  of  volts,  a  certain  definite  amount  of  work 
will  be  required,  which  is  measured  in  volt-coulombs. 
When  this  same  quantity  at  high  pressure  is  allowed  to 
decrease  its  pressure  again,  that  same  amount  of  work  in 
volt-coulombs  will  again  be  given  off,  just  as  in  the  case  of 
the  air.  A  volt-coulomb  is  often  called  a  joule. 

As  in  the  case  of  air,  if  the  quantity  of  electricity  be 
doubled,  the  work  will  be  doubled,  or  if  the  pressure  be 
doubled  the  work  will  be  doubled  ;  in  other  words,  the 
work  in  electrical  units  will  be  the  number  of  volts  pressure 
through  which  it  has  been  raised  or  through  which  it  has 
fallen,  multiplied  by  the  quantity  in  coulombs  which  have 
been  subjected  to  this  change  of  pressure.  The  electrical 
energy  is,  therefore,  merely  the  number  of  volts  multiplied 
by  the  number  of  coulombs. 

The  same  equality  exists  between  electrical  and  mechani- 
cal power.  As  a  horse-power  is  merely  a  certain  number 
of  foot-pounds  of  work  done  per  second,  so  electrical  power 
is  a  certain  number  of  volt-coulombs  of  work  done  per  sec- 
ond. As  a  coulomb  of  electricity  flowing  per  second  is  an 
ampere  of  current,  it  follows  that  a  volt-coulomb  of  work 
per  second  is  a  volt  ampere,  or,  as  it  is  frequently  called,  a 
watt,  from  which  it  follows  that  volt-amperes,  or  watts, 
and  horse-powers  are  units  of  precisely  the  same  kind  and 
may  be  reduced  from  one  to  the  other. 

In  order,  therefore,  to  calculate  the  electrical  power  in 
a  circuit,  lamp,  or  machine,  it  is  only  necessary  to  multiply 
the  volts  pressure  by  the  amperes  of  current.  This  law, 
combined  with  Ohm's  law,  gives  two  more  equally  important 
laws  in  regard  to  electrical  power.  When  the  electromo- 
tive force  and  the  resistance  are  given,  the  power  in  volt- 
amperes  or  watts  is  equal  to  the  square  of  the  electromotive 
force  divided  by  the  resistance  ;  i.  e.,  if  with  the  same 


Review  of  Electrical  Units  and  Fundamental  Laws.  9 

electromotive  force  the  resistance  is  doubled,  the  power  is 
halved  ;  or  if  with  the  same  resistance  the  electromotive 
force  is  doubled,  the  power  is  increased  four  times.  The 
other  law  is  that  when  the  current  and  resistance  are  given, 
the  power  in  volt-amperes  or  watts  is  equal  to  the  square 
of  the  current  multiplied  by  the  resistance.  That  is,  if, 
with  the  same  current,  the  resistance  is  doubled,  the  power 
is  doubled,  or  if,  with  the  same  resistance,  the  current  is 
doubled,  the  power  is  increased  four  times. 

This  last  law  is  often  stated  in  a  very  misleading  way 
in  text  books  and  by  many  scientists,  whose  lack  of 
clearness  in  statement  is  often  very  annoying  to  practical 
engineers.  Thus  we  find  it  stated  that  the  energy  is  pro- 
portional to  the  square  of  the  current.  This  is  not  only 
misleading,  but  in  many  cases  absolutely  false.  The  cur- 
rent, may  be  increased,  while  the  total  energy  may  be 
the  same,  or  greater,  or  less,  depending  on  the  electromotive 
force  or  the  resistance.  The  current  itself  is  no  criterion, 
for  it  might  as  well  be  said  that  the  current  of  water  in  a 
river  represents  horse-power.  If  the  fall  of  the  water  is  not 
taken  into  account,  or  in  electricity  the  electromotive  force 
or  resistance,  the  current  by  itself  is  no  measure  of  the 
power  developed.  It  will  be  readily  seen  that  if  the  cur- 
rent be  doubled  by  halving  the  resistance  (the  electromo- 
tive force  remaining  the  same),  the  energy  will  be  increased 
twice  as  much,  and  not  four  times.  In  another  case,  if  the 
electromotive  force  be  reduced  to  one-quarter  and  the  re- 
sistance reduced  to  one-eighth,  the  current  would  evidently 
be  doubled,  while  the  energy  would  really  be  only  half  as 
great. 

The  law  just  given  is  correct  only  in  one  case,  that  is, 
when  the  resistance  remains  the  same.  The  power  then 
increases  with  the  square  of  the  current,  for  in  order  to 
double  the  current  in  the  same  resistance,  it  is  necessary  to 
double  the  electromotive  force,  thus  doubling  both  volts 
and  amperes,  quadruples  the  volt-amperes. 

The  so-called  absolute  system  furnishes  us  with  a  means 


10  Principles  of  Dynamo-Electric  Machines. 

of  finding  the  actual  quantities  of  foot-pounds  or  horse- 
powers which  correspond  to  a  volt-coulomb  or  volt-ampere. 
These  values  are  as  follows  : 2 

1  volt-coulomb  =       .73732  foot-pounds. 

1  foot-pound     =     1.3563  volt-coulombs,  or  joules. 

1  watt  =       .0013406  horse-power. 

1  horse-power  =  745.941  volt-amperes,  or  watts. 

If  a  pound  weight  be  dropped  freely  through  a  distance 
of  1,000  feet,  or  if  1,000  pounds  be  dropped  through  a 
distance  of  1  foot,  the  quantity  of  work  will  be  precisely 
the  same  in  each  case,  namely,  1,000  foot-pounds.  But  at 
the  same  time  the  quality  or  kind  of  work  will  be  quite 
different.  Or  if  in  one  case  a  small  quantity  of  air  under 
high  pressure,  and  in  another  case  a  large  quantity  of  air 
under  low  pressure,  be  allowed  to  escape,  the  actual  amount 
of  work  done  might  be  the  same,  yet  the  quality  or  kind  of 
work  or  effect  would  be  quite  different.  Similarly,  if  a 
coulomb  of  electricity  is  allowed  to  change  its  potential 
suddenly  1,000  volts,  or  if  1,000  coulombs  be  allowed  to 
fall  through  a  potential  of  one  volt,  the  actual  quantity  of 
work  in  volt-coulombs  will  also  be  the  same  ;  yet  in  the 
two  cases  the  form  or  quality  of  the  energy  dissipated 
would  be  quite  different.  Many  electrical  phenomena  in 
which  electrical  energy  is  dissipated  can,  therefore,  be 
much  more  readily  understood  by  comparing  them  to  air 
under  compression,  the  pressure  being  proportional  to  the 
electrical  pressure  and  the  quantity  proportional  to  the 
quantity  of  electricity.  This  will  readily  answer  the  ques- 
tion frequently  asked,  "  what  is  the  difference  between  the 
electricity  of  a  friction  or  influence  machine,  or  Rhumkorff 
coil,  or  lightning,  and  that  produced  by  batteries,  dynamos, 
and  used  in  telegraphy,  telephony  and  electric  lighting  ?  " 
The  difference  is  merely  in  the  form  or  quality  of  the  energy 
dissipated. 

2.  For  the  method  of  calculating  them,  and  for  a  complete  set  of  equivalents, 
see  THE  ELECTRICIAN,  vol.  ii.,  p.  103  (April,  1883,  New  York) ;  also  Appendix  V. 


Review  of  Electrical  Units  and  Fundamental  Laivs.         11 

There  is  also  a  similarity  between  the  different  forms  in 
which  power  exists.  For  instance,  if  a  pound  weight  be 
dropped  freely  through  a  distance  of  1,000  feet,  or  if  it  be 
allowed  to  fall  slowly,  as  when  driving  a  clock ;  in  both 
cases  the  work  will  be  the  same,  but  as  the  time  in  which  it 
is  done  in  the  two  cases  is  quite  different,  the  power  will  be 
different.  In  the  same  way,  if  a  quantity  of  elastic  gas 
confined  under  high  pressure  be  allowed  to  escape  sud- 
denly, as  in  firing  a  cannon,  or  allowed  to  escape  slowly,  as 
in  a  small  compressed  air  engine,  the  quantity  of  work  may 
be  the  same,  yet  the  kind  or  quality  will  be  very  different. 
Similarly,  if  a  quantity  of  electricity  at  high  pressure  or 
electromotive  force  be  allowed  to  dissipate  its  energy  sud- 
denly, as  in  a  spark  or  bolt  of  lightning,  or  if  it  be  con- 
sumed gradually,  as  in  the  telegraph  or  electric  light,  the 
quantity  in  volt-coulombs  may  be  the  same,  yet  the  power 
in  volt-amperes  will  be  quite  different. 

In  still  another  case  the  power  might  be  the  same  in  the 
two  cases,  yet  its  quality  or  kind  may  be  very  different. 
For  instance,  a  very  high  waterfall  with  small  quantity  of 
water,  on  the  one  hand,  and  a  sluggishly  flowing  river  with 
small  fall,  on  the  other  hand,  may  each  develop  precisely 
the  same  available  horse-power,  yet  the  quality  of  their 
power,  and  therefore  the  apparatus  necessary  to  effect  its 
conversion,  will  be  quite  different.  This  is  analogous  to  a 
dynamo  of  high  electromotive  force  and  small  current, 
such  as  is  used  for  arc  lighting,  and  another  of  low  electro- 
motive force  and  great  current,  such  as  is  used  for  electro- 
plating ;  they  may  both  require  the  same  horse-power,  yet 
the  form  of  the  electrical  power  is  different. 

A  few  quantitative  examples,  illustrating  what  has  been 
said  above,  may  be  of  interest.  The  pressure  of  ignited 
gunpowder  being  about  40  tons  per  square  inch,  the  energy 
in  a  bullet  fired  from  a  small  pistol  is  about  600  foot- 
pounds. As  a  pocket  watch  consumes  about  one  fifty-four 
millionth  of  a  horse-power,  it  follows  that  the  amount  of 
energy  in  the  bullet  would  run  the  watch  almost  two  years. 


12  Principles  of  Dynamo-Electric  Machines. 

As  the  current  in  an  Edison  telephone  transmitter  was 
found  to  be  about  .0001  ampere,3  and  its  resistance  about 
1  ohm,  the  energy  in  it  is  slightly  less  than  one-thousandth 
part  of  that  used  in  a  watch,  and  it  would  therefore  be  run 
over  2,000  years  with  this  same  amount  of  energy  as  devel- 
oped in  a  small  pistol. 

The  charge  of  a  small  Leyden  jar  was  found  to  be 
.000,008  coulombs  at  about  50,000  volts.  This  in  discharg- 
ing would  develop  .4  volt-coulombs,  or  .288  foot-pounds  of 
energy,  which  would  run  a  watch  about  eight  hours,  and 
is  about  equal  to  double  the  amount  of  energy  consumed 
per  second  in  the  line  of  a  telegraph  circuit  of  100  miles. 

Several  authorities  have  calculated  the  electromotive 
force  of  a  bolt  of  lightning  to  be  about  3,500,000  volts, 
and  the  current  to  be  about  14,000,000  amperes,  the  time 
of  the  bolt  being  measured  to  be  one  twenty-thousandth  of 
a  second.  A  simple  calculation  shows  that  this  amount  of 
energy  set  free  in  that  short  time  is  about  as  much  as  that 
of  a  100  h.  p.  engine  for  almost  10  hours. 

Besides  the  electrical  units  above  mentioned,  there  are 
certain  others  less  frequently  used. 

Capacity  in  electrotechnics  is  very  similar  to  capacity  in 
mechanics.  Although  it  cannot  be  measured  in  cubic 
inches  or  quarts,  yet  its  function,  which  is  to  measure 
quantity,  is  the  same  as  in  the  ordinary  sense.  Just  as  a 
quart  measure  may  hold  a  certain  amount  of  air  at  at- 
mospheric pressure,  or  double  that  amount  (by  weight)  at 
two  atmospheres,  and  so  on,  so  does  an  electrical  condenser, 
having  a  certain  electrical  capacity,  hold  a  certain  definite 
amount  of  electricity  at  a  certain  electrical  pressure  or 
electromotive  force,  and  double  this  amount  at  double  the 
pressure,  and  so  on.  Any  amount  of  electricity  can  be 
forced  into  a  condenser  of  a  given  capacity  by  increasing 
the  pressure  in  proportion,  in  the  same  way  that  any  quan- 
tity of  air  (by  weight)  can  be  forced  into  a  quart  measure 
by  simply  increasing  the  pressure  accordingly. 

3.  See  ELECTRICIAN  AND  ELECTRICAL  ENGINEER,  Nov.,  1885,  vol.  iv.,  p.  422. 


Review  of  Electrical  Units  and  Fundamental  Laws.         13 

From  this  the  law  in  regard  to  capacity  becomes  self- 
evident.  The  quantity  of  electricity  in  coulombs  in  a 
given  condenser,  divided  by  its  pressure  in  volts,  is  always 
a  constant,  and  is  equal  to  the  capacity  of  the  condenser 
in  farads  or  units  of  capacity. 

Two  other  laws  follow  from  this,  namely,  that  the  quan- 
tity of  electricity  in  a  condenser  is  the  capacity  multiplied 
by  the  pressure  or  electromotive  force.  And  lastly,  the 
pressure  or  electromotive  force  of  the  electricity  in  a  con- 
denser is  equal  to  the  quantity  of  electricity  divided  by  the 
capacity  of  the  condenser. 

Ampere-hour  is  a  convenient  term  for  representing  3,600 
coulombs,  as  it  is  equal  to  an  ampere  flowing  for  one  hour, 
and  is  used  in  connection  with  the  accumulation  of  elec- 
trical power  in  batteries. 

Ampere-winding  or  ampere-turn,  used  in  reference  to 
electro  magnets  and  solenoids,  represents  an  ampere  of 
current  circulating  once  around  the  coil.  The  number  of 
windings  or  turns  of  wire  in  a  coil,  multiplied  by  the  cur- 
rent in  the  wire,  gives  the  number  of  ampere-turns  of  the 
whole  coil.  As  will  be  seen  hereafter,  it  is  a  term  used  for 
representing,  measuring  and  calculating  the  magnetism  of 
machine  magnets. 

Electrical  horse-power  is  merely  the  number  of  volt- 
amperes  which  equal  a  mechanical  horse-power,  as  given 
in  the  table  on  page  10. 


CHAPTER   H. 

Fundamental  Principles  of  Dynamos  and  Motors. 

THE  explanations  which  are  given  in  many  text-books  of 
the  process  of  generating  electric  currents  in  the  dynamo 
machine,  and  the  production  of  mechanical  power  in  the 
electric  motor  by  means  of  the  electric  current,  are  in  many 
cases  unsatisfactory.  This  is  largely  due  to  the  failure  of 
the  authors  to  lay  down  any  fundamental  principle,  applic- 
able alike  to  all  kinds  of  machines.  The  principles  involved 
are  often  explained  in  an  indirect  and  circuitous  way,  and 
in  one  well-known  work  recently  published,  the  explana- 
tion of  the  manner  in  which  the  currents  are  generated  in 
a  Gramme  ring  armature  are  not  merely  misleading  but 
absolutely  erroneous,  and  are  in  fact  inconsistent  with  actual 
results  which  may  be  obtained  even  from  a  rough  test. 

In  attempting  an  explanation  of  the  principle  of  the 
dynamo,  we  propose  therefore,  to  discard  insufficient  and 
erroneous  theories,  and  to  take  for  our  starting  point  the 
fundamental  law  illustrated  in  the  action  between  a 
magnet  and  a  current,  as  conclusively  demonstrated  by  one 
of  the  most  simple  of  experiments.  By  so  doing  it  will 
be  found  that  the  generation  of  currents  and  of  power  in 
dynamos  and  motors  may  be  explained  by  simple  and  direct 
methods,  while  at  the  same  time  the  general  directions 
which  should  be  pursued  to  obtain  the  most  economical 
proportions  are  pointed  out. 

In  the  year  1819,  Hans  Christian  Oersted,  a  Danish 
philosopher,  discovered  that  a  current  of  electricity  when 
caused  to  pass  near  a  magnetic  needle  will  deflect  it.  Or, 
in  other  words,  he  discovered  that  an  electric  current  and 
a  magnet  exert  a  mutual  force  upon  each  other.  This 
simple  experiment  is  a  demonstration  of  the  fundamental 
principle  of  all  dynamos  and  motors  ;  by  its  means,  the 

(14) 


Fundamental  Principles  of  Dynamos  and  Motors.  15 

generation  of  currents  or  power  in  all  the  different  types 
of  machines  may  be  rendered  easily  intelligible  without 
the  intervention  of  abstruse  or  complicated  theories,  which 
in  many  cases  have  to  be  in  themselves  varied  to  correspond 
with  the  facts  observed  in  various  types  of  machines. 

All  electric  motors  are  nothing  more  than  direct  appli- 
cations of  this  principle  on  a  large  scale,  for  whatever  is 
true  of  a  small  magnetic  needle  and  a  weak  current,  is 
equally  true,  in  principle,  of  a  large  magnet  and  a  powerful 
current.  Every  motor  will  be  found  upon  analysis  to  con- 
sist of  one  or  more  magnets  and  a  conductor  traversed  by 
a  current  of  electricity,  one  of  the  two,  or  both,  being 
capable  of  receiving  a  continued  motion  from  the  mutual 
force  exerted  between  the  magnet  and  the  current,  as 
shown  in  Oersted's  experiment.  The  movable  conductor 
may  be  in  the  form  of  one  or  more  straight  wires,  or  it  may 
be  a  coil,  or,  as  in  a  unipolar  motor,  a  disc  or  hollow  cylin- 
der. In  each  of  these  modifications  the  operative  principle 
remains  the  same,  that  is,  simply  the  mutual  action  be- 
tween the  magnet  and  the  current. 

In  electric  motors  this  principle  applies  directly,  as 
these  are  merely  practical  devices  for  carrying  out  Oersted's 
experiment  on  a  large  scale  under  such  mechanical  condi- 
tions as  to  obtain  constant  rotary  motion.  In  the  case  of 
dynamos,  however,  a  deduction  from  this  principle  applies, 
for  while  in  motors  the  exact  conditions  of  Oersted's 
experiment  exist,  in  generators  the  magnet  and  the 
mechanical  force  are  given,  the  resulting  current  being 
that  which  is  required.  The  explanation  of  the  action  of 
generators,  however,  offers  no  additional  difficulties,  as  the 
converse  of  Oersted's  experiment  is  also  true,  namely,  that 
if  the  conditions  of  that  experiment  be  reversed,  by  mov- 
ing the  conductor  in  the  reverse  direction,  a  current  will  tend 
to  be  generated  in  that  conductor,  or,  as  it  is  generally 
stated,  a  current  will  be  induced  in  that  conductor,  which 
is  then  termed  the  inductor.  This  is  commonly  known  as 
one  of  Lenz's  laws,  which,  stated  in  more  precise  terms,  is 


16  Principles  of  Dynamo- Electric  Machines. 

this  :  If  a  conductor  be  moved  near  a  magnet  it  tends  to 
generate  a  current,  which  will  flow  in  the  opposite  direction 
to  the  current  which  would,  by  its  action  on  the  magnet, 
produce  that  motion. 

This  may  be  further  illustrated  by  analogy.  Between 
the  earth  and  a  body  held  at  some  distance  from  it,  there 
exists  a  force,  which  if  allowed  to  act,  will  draw  the  body 
to  the  earth.  Now  if  this  is  reversed  by  overcoming  the 
force  and  lifting  the  weight,  the  amount  of  energy  which 
was  used  in  lifting  it  will  again  be  stored  up  in  the 
weight.  Or,  in  other  words,  if  the  process  of  the  falling 
weight  is  reversed,  the  same  energy  which  was  set  free  in 
falling  will  be  generated  again  in  the  body.  Similarly  if 
Oersted's  experiment,  or  the  electric  motor,  be  reversed 
by  moving  the  conductor  near  the  magnet  in  the  opposite 
direction,  the  same  amount  of  current  will  be  generated 
that  would  have  been  necessary  to  produce  this  motion. 
As  in  the  case  of  the  weight,  in  which  the  energy  developed 
in  falling  is  equal  to  that  required  to  raise  it,  so  in  electric 
induction,  the  current  generated  by  moving  the  wire 
is  theoretically  equal  to  that  which  is  required  to  produce 
that  motion.  For  instance,  if  a  certain  amount  of  electri- 
cal energy  in  a  movable  conductor  exerts  a  force  or  ten- 
dency to  move  in  a  certain  direction,  equal  to  one  pound, 
on  a  large  magnet,  then  it  would  require  a  force  of  one 
pound  to  generate  that  same  amount  of  electrical  energy 
in  that  conductor  by  moving  it  in  the  opposite  direction. 

These  are  the  fundamental  principles  underlying  the 
construction  of  all  dynamos  and  motors,  and  by  means  of 
which  the  action  in  all  of  them  may  be  explained,  be  they 
constant  current,  alternating  current,  unipolar,  bi-polar  or 
multi-polar.  They  all  consist  of  a  magnet  and  a  conduc- 
tor for  the  current,  one  or  the  other,  or  both  of  which  are 
movable,  as  in  Oersted's  experiment. 

In  the  explanation,  just  given,  it  was  said  that  a  cur- 
rent is  generated  by  moving  a  wire  in  the  vicinity  of  a 
magnet.  It  must  be  remembered,  however,  as  pointed 


Fundamental  Principles  of  Dynamos  and  Motors.  17 

out  in  the  first  chapter,  that  a  current  is  merely  the 
result  of  the  equalization  of  a  difference  of  electrical 
pressure.  Strictly  speaking,  therefore,  it  is  not  actually 
current  but  electromotive  force  or  electrical  pressure, 
which  is  generated  by  the  induction  in  the  moving 
wire,  for  by  disconnecting  or  opening  the  circuit,  it 
will  be  found  that  the  electromotive  force  still  exists, 
even  if  no  current  can  flow.  The  word  current  was 
used  merely  for  the  sake  of  avoiding  a  multiplicity  of 
terms  in  the  explanation.  A  current  does  exist  in  most 
cases,  but  its  existence  is  due  only  indirectly  to  induction. 
Its  actual  origin  is  due  to  an  equalization  of  the  electro- 
motive force  which  has  been  generated  by  induction.  As 
pointed  out  in  the  first  chapter,  it  is  quite  similar  to  the 
case  of  a  centrifugal  air-blower,  in  which  the  pressure  is 
that  which  is  generated,  the  current  being  only  a  second- 
ary  result  of  this  pressure.  This  conception  of  induction 
will  be  found  to  materially  simplify  the  conception  and 
solution  of  many  problems  which  are  met  with  in  practice, 
as,  for  instance,  the  determination  of  the  most  advantage- 
ous mode  of  insulating  the  iron  of  armatures  to  avoid 
Foucault  currents,  in  which  case  it  is  the  current  which 
must  be  avoided,  not- the  generation  of  the  difference  of 
potential,  for  that  cannot  possibly  be  avoided  except  by 
an  absence  of  all  material,  or  by  keeping  the  iron  fixed  in 
one  position. 

With  a  magnetic  field  of  known  strength,  and  with  a 
given  speed  and  direction  of  motion  of  the  inductor,  a 
certain  definite  electromotive  f6rce  is  developed,  which 
may  be  theoretically  calculated.  But  it  is  evident  that 
with  this  electromotive  force  any  desired  current  may  be 
generated,  according  to  the  resistance  of  the  circuit  through 
which  this  electrical  pressure  equalizes  itself,  showing  that  i 
the  current  is  directly  dependent  only  on  the  electromotive 
force  and  resistance,  and  not  on  the  magnet  nor  the  speed. 


CHAPTER   III. 

Magnetism  and  Electro-Magnetic  Induction. 

AMONG  the  different  units  and  expressions  used  in  repre- 
senting and  measuring  magnetism  and  magnetic  forces, 
and  the  laws  by  which  these  are  governed,  the  most 
important  are  the  following. 

Lines  of  force  are  units  in  terms  of  which  magnetism 
may  be  expressed  and  measured.  A  magnetic  field  which 
is  nothing  more  than  a  space  in  which  magnetism  exists, 
may  be  said  to  contain  lines  of  force.  The  direction  and 
polarity  of  these,  indicate  the  direction  and  polarity  of  the 
magnetic  force  ;  their  total  number  is  a  measure  of  the 
amount  of  magnetism  ;  while  the  number  per  square  inch 
of  area,  measured  perpendicularly  to  their  direction,  is  a 
measure  of  the  intensity  of  magnetism  at  that  point. 

This  conception  of  magnetic  force  may  perhaps  be  better 
understood  if  compared  to  the  force  of  gravity  similarly 
represented.  Imagine  a  heavy  body  suspended  in  the  air, 
and  suppose  every  cubic  inch  of  the  material  of  which  the 
body  is  composed,  to  weigh  one  pound.  If  an  imaginary 
line  be  drawn  to  the  earth  from  the  centre  of  gravity  of 
each  cubic  inch  of  the  suspended  body,  the  direction  of 
these  lines  would  represent  the  direction  of  the  force  of 
gravity  ;  their  total  number  would  represent  the  total  force 
in  pounds,  while  their  density,  or  the  number  of  lines  per 
square  inch  area  (measured  perpendicularly  to  their  direc- 
tion) would  represent  the  intensity  of  the  force  at  that 
point.  In  precisely  the  same  way  as  these  lines  represent 
the  direction,  amount,  and  intensity  of  the  force  of  gravity 
in  that  body,  so  do  the  lines  of  magnetic  force  represent 
the  direction,  amount,  and  intensity  of  magnetism,  except 
that  in  the  latter  there  is  no  constant  direction  of  action 
such  as  the  downward  force  of  gravity,  lines  of  force 

(18) 


Magnetism  and  Electro-Magnetic  Induction.  19 

acting  in  both  directions  as  if  trying  to  shorten  their 
circuit,  like  a  stretched  rubber  ring.  The  lines  do  not 
exist  as  such,  any  more  than  they  do  in  the  analogy  of  the 
force  of  gravity,  it  is  merely  a  convenient  way  of  repre- 
senting magnetism  in  order  to  facilitate  the  conception  and 
computation  of  problems. 

The  amount  or  quantity  of  magnetism  is  therefore  ex- 
pressed by  the  total  number  of  lines  of  force.  Magnetic 
density,  or  intensity  of  magnetism  is  expressed  by  the  num- 
ber of  lines  of  force  per  square  inch  area  measured  perpen- 
dicularly to  their  direction.  The  polarity  of  a  magnet  or 
a  magnetic  field,  is  represented  by  the  direction  of  the  lines 
of  force  (indicated  by  arrow  heads),  as  distinguished 
from  their  position.  The  axis  of  magnetization  is  a  line 
parallel  to  the  path  of  the  lines  of  force. 

A  single  line  of  force,  or  unit,  is  that  amount  of  mag- 
netism which  passes  through  every  square  centimetre  of 
cross  section  of  a  magnetic  field  whose  intensity  is  unity. 
Such  a  magnetic  field  exists  at  the  centre  of  curvature  of  an 
arc  of  a  circle,  whose  radius  is  one  centimetre,  and  whose 
length  is  also  one  centimetre,  when  a  current  of  10  amperes 
flows  through  this  arc.  Or,  stated  in  the  language  of  the 
practical  electrician,  it  is  the  amount  of  magnetism  which 
passes  through  an  area  of  one  square  centimetre,  at  the  centre 
of  a  coil  of  one  turn,  having  a  diameter  of  10  centimetres, 
when  a  current  of  about  eight  amperes  (accurately  7.9578) 
flows  through  the  wire.  The  intensity  of  magnetization  in 
the  area  enclosed  by  a  circular  coil  is  different  in  different 
parts,  being  least  in  the  center  and  strongest  nearest  the  cir- 
cumference. 

A  uniform  field,  or  field  of  uniform  intensity,  is  one  in 
which  the  number  of  lines  of  force  per  square  centimeter 
is  the  same  throughout  the  field.  The  best  illustration 
of  a  uniform  field  is  the  magnetic  field  of  the  earth  at 
any  one  place.  The  intensity  of  the  earth's  field  in  the 
direction  of  the  "dip"  is  about  .5,  which  means  that  through 
every  square  centimeter  measured  perpendicularly  to  the 


20  Principles  of  Dynamo-Electric  Machines. 

direction  of  magnetization,  -J  of  a  line  of  force  passes,  or 
in  other  words,  through  every  two  square  centimetres  one 
line  of  force  passes. 

The  direction  assumed  for  lines  of  force  (generally  indi- 
cated by  arrows),  as  distinguished  from  their  position,  is, 
of  course,  arbitrary,  as  they  have  no  such  direction  of 


FIGURE  1. 

action,  as  mentioned  before,  but  in  practice  it  is  very  con- 
venient for  defining  terms  and  for  stating  laws,  to  concede 
them  to  have  a  certain  direction,  and  it  has  become  uni- 
versal to  consider  them  as  emanating  from  the  north  pole 
of  a  magnet  and  entering  the  south  pole,  as  shown  in  fig- 
ure 1.  Thus  the  direction  of  the  lines  of  force  in  any  mag- 
netic field  would  be  the  direction  in  which  a  magnetic 


Magnetism  and  Electro-Magnetic  Induction.  21 

needle  M,  figure  1,  points,  if  made  as  usual,  in  the  form  of 
an  arrow  having  the  point  at  the  north  (north  seeking)  end. 
An  electric  current  is  always  surrounded  by  lines  of 
force  which  encircle  it,  their  density  decreasing  as  their 
distance  from  the  current  increases,  as  shown  in  figure  2. 
Assuming  that  the  lines  of  force  have  a  direction  as  just 
described,  then  if  the  current  flows  away  from  the  observer, 
the  line  of  force  will  pass  around  in  the  same  direction 


FIGURE  2. 


as  that  of  the  hands  of  a  watch,  as  will  be  seen  in  figure  2. 
In  other  words,  below  the  wire  the  north  pole  will  be  to 
the  left  and  the  south  pole  to  the  right,  while  the  reverse 
is  the  case  above  the  wire. 

From  this  the  laws  of  the  polarity  of  electro-magnets 
follow.  In  looking  at  the  face  of  the  pole,  if  the  current 
flows  in  the  direction  of  the  hands  of  a  watch,  it  will  be  a 
south  pole,  and  if  in  the  other  direction  it  will  be  a  north 
pole,  as  illustrated  in  figure  3,  which  shows  some  of  the 
lines  of  force  encircling  the  wires  of  the  coil  and  conden- 
sing in  the  poles  of  the  magnet.  This  rule  is  easily  remem- 
bered, as  it  is  the  same  direction  as  that  of  the  rotation  of 
the  earth  in  looking  at  its  geographic  north  and  south  poles 
from  points  in  space. 

Lines  of  force  which  have  the  same  direction  repel  each 
other,  while  if  they  have  opposite  directions  they  attract 
each  other.  This  explains  why  like  currents  of  electricity 
attract,  and  unlike  repel  each  other,  as  will  be  seen  in  fig- 


22 


Principles  of  Dynamo- Electric  Machines. 


ure  4,  in  which  two  like  and  two  unlike  currents  are  shown 
with  some  of  their  lines  of  force  encircling  them  ;  in  the 
first  two  the  neighboring  lines  of  force  are  unlike,  in  the 
last  two  they  are  like. 


FIGURE  3. 

The  following  properties  of  lines  of  force  and  of  magnets, 
will  frequently  be  found  useful  in  designing  machine  mag- 
nets. Every  line  of  force  makes  a  complete  closed  circuit, 
that  is,  it  emanates  from  a  north  pole,  passes  through  the 
field  around  to  the  south  pole  and  returns  to  the  north  pole 
again  through  the  magnet.  From  this  it  follows  that  the 
number  of  lines  of  force  is  the  same  in  all  the  cross  sections 
of  their  complete  circuit,  and  therefore  the  intensity  of  a 
field  is  inversely  proportional  to  its  cross  section  if  it  is 
uniform,  showing  that  the  most  intense  field  is  in  the  iron 
of  the  magnet  where  it  cannot  be  utilized.  It  also  follows 
from  this  that  around  a  single  pole  the  intensity  of  the  field 
(number  of  lines  of  force  per  unit  area)  diminishes  as  the 
square  of  the  distance  to  the  pole. 

Magnetic  materials  readily  conduct  and,  therefore,  also 
condense  in  them,  lines  of  force2  while  non-magnetic  ma- 
terials offer  great  resistance  to  them. 

2.  It  is  claimed  by  some  that  iron  merely  conducts  and  condenses  magnet- 
ism, but  has  no  inherent  magnetism  of  its  own.  It  is  very  doubtful  whether  this 
last  statement  is  correct. 


Magnetism  and  Electro-Magnetic  Induction.  23 

In  applying  these  properties  of  lines  of  force  to  dynamos, 
it  follows  that  all  the  parts  of  their  circuit  should  offer  the 
least  possible  resistance  to  them.  For  instance  in  the 
Weston  type  of  frame,  with  four  coils,  the  cross  section  of  the 
end  pieces  should  be  at  least  equal  to  that  of  the  core  of  a 
magnet  ;  that  of  the  pole  pieces  measured  perpendicularly  to 
the  direction  of  the  lines  of  force,  should  be  at  least  double 
that  of  a  core  ;  the  iron  of  the  armature,  measured  hori- 
zontally should  have  at  least  twice  the  cross  section  of  a 
core,  if  it  were  made  of  the  same  material ;  and  as  the  space 
between  the  armature  and  pole  pieces  offers  the  greatest 
resistance,  it  should  be  made  as  short  as  possible  in  the 


FIGURE  4. 


direction  of  the  lines  of  force  and  have  as  large  a  cross  sec- 
tion perpendicular  to  this  direction,  as  other  considerations 
will  permit.  The  average  thickness  of  this  space  may  be 
reduced  by  projecting  iron  lugs  between  the  wires  (like  in 


24  Principles  of  Dynamo- Electric  Machines. 

a  Pacinotti  ring  armature  as  distinguished  from  a  Gramme), 
which  S.  P.  Thompson  has  shown  increases  the  effect.3 

In  regard  to  the  magnetic  properties  of  wrought  and 
cast-iron,  a  certain  authority  states  that  for  the  same  coil 
and  current  the  magnetism  of  a  wrought  iron  core  is  80 
per  cent,  greater  than  that  of  a  cast  iron  one.  From  this 
it  follows  that  if  the  cost  of  the  wire  and  iron  of  a  30  per 
cent,  larger  cast  iron  magnet  is  less  than  that  of  the 
smaller  wrought  iron  one  of  equal  strength,  then  the 
former  should  be  used,  notwithstanding  its  increased  si/e 
and  its  poorer  magnetic  properties.  The  relative  cost  of 
wrought  iron  and  cast  iron  magnets  depends  evidently  on 
the  style  of  the  frame  of  the  machine.  S.  P.  Thompson 
states  that  a  cast  iron  magnet  has  only  60  per  cent,  of  the 
effect  of  a  wrought  iron  one  of  the  same  size.  He  also 
states  that  the  permeability  of  iron  to  magnetic  lines  of 
force  is  from  40  to  20,000  times  that  of  the  air,  from 
which  it  would  follow  that  the  cross  section  of  an  air 
space  in  the  circuit  of  the  lines  of  force,  measured  perpen- 
dicularly to  them,  would  have  to  be  40  to  20,000  times 
that  of  the  iron  to  offer  the  same  relative  magnetic  resist- 
ance. 

Lines  of  force  can  never  intersect  each  other  ;  when  a 
number  act  at  the  same  point,  in  different  directions  (as 
in  a  galvanometer  in  the  earth's  field,  or  in  the  combined 
field  of  an  armature  and  its  field  magnets),  their  total  ac- 
tion will  be  represented  in  intensity  and  direction  by  their 
single  resultants.  If  such  a  resultant  is  allowed  to  act, 
as  in  the  case  of  a  magnetic  needle  containing  fixed  lines 
of  force  in  it,  suspended  so  as  to  be  free  to  move,  then  the 
effect  will  be  to  move  the  needle  until  this  resultant 
passes  through  the  centre  at  which  the  needle  is  suspended. 
This  illustrates  the  law  that  lines  of  force  strive  to  ar- 
range themselves  parallel  to  one  another. 

Lines  of  force  act  as  though  they  were  elastic,  tending 

3.  Dynamo  Electric  Machinery,  1st  edition,  p.  6(M>9. 


Magnetism  and  Electro-Magnetic  Induction.  25 

to  make  their  complete  circuit  as  short  as  possible,  and 
exerting  a  lateral  repelling  force  upon  one  another,  thus 
tending  to  equalize  the  magnetic  density  in  a  homogen- 
eous medium.  The  resultant  of  these  two  actions  at  dif- 
ferent points  determines  their  position  and  density,  as 
illustrated  in  the  field  around  a  bar  magnet  by  the  well- 
known  experiment  of  throwing  iron  filings  on  a  glass 
plate  covering  the  magnet. 

Some  of  the  general  laws  of  electro-magnetic  induction 
are  as  follows  : 

The  electromotive  force  generated  by  moving  a  con- 
ductor in  a  magnetic  field,  is  proportional  to  the  number 
of  lines  of  force  cut  per  second  or  to  the  rate  of  cutting  lines 
of  force.  It  therefore  increases  with  the  speed  of  the 
moving  wire  and  with  the  number  of  lines  of  force  cut, 
but  not  with  the  size  of  the  field  passed  through  per  sec- 
ond without  increasing  at  the  same  time  the  total  number 
of  lines  of  force.  It  follows  from  this  that  the  greatest 
effect  is  produced  by  cutting  them  perpendicularly,  for 
then  the  number  passed  through  in  the  same  distance  of 
motion  is  greatest.  It  also  follows  that  moving  the  con- 
ductor in  the  direction  of  the  lines  of  force  generates  no 
potential,  as  in  that  case  none  are  cut. 

By  definition,  one  volt  electromotive  force  is  generated 
for  every  100,000,000  lines  of  force  cut  per  second,  or  in 
other  words,  the  electromotive  force  in  volts  generated  in 
the  inductor  is  equal  to  the  total  number  of  lines  of  force 
cut,  divided  by  100,000,000  and  by  the  time  in  seconds  in 
which  it  passes  through  them. 

The  direction  of  the  motion  in  motors,  or  the  direction 
of  the  current  in  generators,  may  be  determined  by  the 
following  "  rules  of  thumb,"  which  are  more  practical  and 
often  less  awkward  in  their  application  than  Ampere's 
well-known  rule  of  "  swimming  in  the  current." 

In  the  conditions  of  Oersted's  experiment  the  letters  of 
the  word  NOSE  give  the  initial  letters  of  the  principal 
words  of  the  rule  "  If  the  current  passes  from  the  JVorth 


26  Principles  of  Dynamo- Electric  Machines. 

pole  Over  the  needle  to  the  $outh,  the  deflection  of  the 
north  end  will  be  toward  the  .East."  Similarly  the  word 
SN~0  W  is  the  key  to  the  rule  "  If  the  current  flows  from 
the  /South  to  the  jVbrth  Over  the  needle,  the  north  end 
will  be  deflected  toward  the  TPest." 

In  applying  these  rules  to  generators  it  need  only  be 
remembered  that  if  the  inductor  be  moved  near  the  mag- 
net, the  current  induced  will  be  in  the  opposite  direction 
to  that  which  would  produce  this  same  motion. 

A  still  better  practical  rule,  and  one  which  is  very  easily 
remembered  is  illustrated  in  figure  5.  If  a  north  pole  of  a 
magnet  be  grasped  in  the  right  hand,  as  illustrated,  and 


FIGURE  5 

the  three  fingers  of  the  hand  be  held  in  the  position  shown, 
then  the  fore  finger  will  point  in  the  direction  of  the  lines 
of  force,  which  are  assumed  to  emanate  from  a  north  pole; 
and  if  the  thumb  points  in  the  direction  of  motion  of  the 
inductor  or  moving  wire,  that  is,  perpendicular  to  the  lines 
of  force,  the  third  finger  will  then  point  in  the  direction  in 
which  the  current  will  tend  to  flow  under  these  conditions. 
In  other  words,  if  a  wire  be  moved  past  a  north  pole  in  the 
direction  in  which  the  thumb  points,  the  induced  current 


Magnetism  and  Electro-Magnetic  Induction.  27 

will  tend  to  flow  in  the  direction  in  which  the  third  finger 
points.  This  is  called  Fleming's  rule.  In  this  as  well  as  in 
the  other  rules,  it  is  evident  that  if  one  of  the  conditions  be 
reversed,  then  the  current,  or  the  motion,  as  the  case  may 
be,  will  be  in  the  opposite  direction,  while  if  two  conditions 
be  reversed,  the  current  or  the  motion  will  be  in  the  same 
direction.  For  instance,  if  a  north  pole  be  grasped  simi- 
larly in  the  left  hand  it  will  give  the  conditions  of  a  motor 
or  of  Oersted's  experiment,  the  thumb  in  that  case  pointing 
in  the  direction  of  the  motion  which  the  wire  (not  the  mag- 
net) will  have  if  the  current  in  it  flows  in  the  direction  as 
indicated  by  the  third  finger. 

In  connection  with  the  above  rules  it  must  be  remem- 
bered, as  before  pointed  out,  that  it  is  the  electromotive 
force  and  not  the  current  which  is  induced,  and  that,  there- 
fore, the  electric  polarity  of  such  a  wire  in  which  induction 
takes  place,  is  the  reverse  of  what  it  would  have  to  be  to 
cause  that  current  to  flow  through  the  conductor.  In  other 
words,  as  the  positive  pole  of  the  inductor  is  the  one  out  oj 
which  the  current  tends  to  flow,  it  follows  that  the  current 
in  the  inductor  itself  flows  from  its  negative  to  its  positive 
pole,  just  as  in  a  battery  in  which  the  current  in  the  liquid 
flows  from  the  negative  (zinc  pole)  to  the  positive  (copper 
or  carbon)  pole,  and  not  from  the  positive  to  the  negative 
as  in  the  external  circuit. 

It  follows  from  the  above  laws  that  with  a  magnetic 
field  of  a  certain  strength,  and  a  moving  inductor  having  a 
given  speed  and  position  in  the  field,  a  certain  definite  elec- 
tromotive force  will  be  generated  which  is  quite  independ- 
ent of  the  resistance  of  the  circuit.  From  Ohm's  law  it 
follows  that  any  desired  current  can  be  generated  with  a 
certain  electromotive  force  as  it  depends  only  on  the  re- 
sistance. Therefore,  it  is  evident  that  any  desired  current 
can  be  generated  with  a  given  magnet  and  a  given  speed 
of  motion  and  position  of  the  inductor,  or  in  other  words 
the  current  is  not  directly  dependent  in  either  the^  magnet 
or  the  speed,  but  is  the  quotient  of  the  electromotive  force 


28  Principles  of  Dynamo- Electric  Machines. 

divided  by  the  total  resistance,  the  latter  quantity  being  in 
most  cases  a  variable  one. 

In  one  sense,  however,  the  electromotive  force  in  an 
electric  machine  as  distinguished  from  batteries,  is  to  a 
certain  extent  dependent  indirectly  on  the  current,  for 
when  it  flows  in  the  inductor  it  tends  in  all  cases  to  oppose 
or  weaken  the  magnet,  and  therefore  indirectly  affects  the 
electromotive  force.  In  other  words  the  counter-magnet- 
ism of  the  armature  of  a  machine  tends  to  weaken  the  field, 
hence  the  practical  rule  :  Make  the  magnetism  of  the  field 
as  strong  as  possible,  and  the  counter-magnetism  of  the 
armature  as  weak  as  possible,  which  is  done  by  making  the 
number  of  windings  on  the  armature  as  small  as  possible. 

It  may  be  well  to  call  attention  here  to  a  distinction 
between  the  terms  "  electromotive  force  "  and  "  difference 
of  potential,"  which  it  is  very  desirable  to  adhere  to  as 
strictly  in  English  as  is  done  in  the  French  and  German 
languages,  as  it  often  aids  greatly  in  clearness  of  statement. 
Difference  of  potential,  is,  as  the  name  implies,  the  differ- 
ence of  the  electrical  potential  at  any  two  points  of  a 
circuit,  and  may  therefore  be  applied  to  that  at  the  poles  of 
a  machine,  battery,  or  lamp,  or  at  the  ends  of  leads,  or  in 
general  to  any  two  points  in  a  circuit.  The  term  "  electro- 
motive force,"  however,  applies  only  to  the  maximum 
difference  of  potential  which  exists  in  the  circuit,  or  in 
other  words,  the  total  generated  difference  of  potential.  It 
applies,  therefore,  only  to  the  generators  of  differences  of 
potential,  such  as  machines  or  batteries  and  to  counter  or 
opposing  differences  of  potential  developed  in  motors,  arc 
lamps,  or  in  the  charging  of  secondary  batteries.  The 
available  difference  of  potential  at  the  poles  of  a  generator 
is  therefore  its  electromotive  force,  less  that  which  is 
absorbed  by  the  internal  resistance  of  the  generator,  and 
which  according  to  Ohm's  law  is  dependent  on  the  current, 
while  the  electromotive  force  is  not  dependent  on  the 
current  (except  indirectly  as  mentioned  above,  as  in  the 
case  of  machines  or  in  polarizing  batteries).  In  generators 


Magnetism  and  Electro-Magnetic  Induction.  39 

the  two  are  equal  when  measured  on  open  circuit  with  an 
electrometer  or  any  other  instrument  in  which  no  current 
needs  to  flow. 


CHAPTER    IV. 

Generation  of  Electromotive  Force  in  Dynamos. 

THE  primary  object  in  a  dynamo  electric  machine  is,  as 
was  explained  in  chapter  ii,  to  generate  an  electromotive 
force  or  electrical  pressure  ;  the  resulting  current,  which 
depends  on  the  resistance  in  the  whole  circuit  through 
which  this  electromotive  force  equalizes  itself,  is,  in  gen- 
eral, of  secondary  importance  in  the  construction  of 
machines.  Just  as  in  the  case  of  a  number  of  batteries  for 
strong  currents,  the  first  consideration  is  to  develop  the 
required  electromotive  force,  which  depends  only  on  the 
chemical  constituents  and  the  number  of  cells  ;  the  cur- 
rent which  they  are  required  to  give  is  of  secondary  im- 
portance, and  affects  only  the  size  of  the  plates  in  permit- 
ting this  current  to  pass  through  the  battery  itself  without 
too  much  loss.  If  a  machine  has  been  constructed  to  give 
a  certain  electromotive  force,  the  greatest  current  which 
it  will  be  able  to  maintain  depends,  in  general,  only  on  the 
thickness  of  the  wire  in  the  machine  through  which  this 
current  has  to  pass.  The  current,  therefore,  affects  the 
construction  only  in  the  detailed  dimensions  of  the 
machine,  which  will  be  discussed  under  the  details  of  con- 
struction, the  present  chapter  being  limited  to  the  methods 
of  generating  the  electromotive  force. 

In  a  dynamo  electric  machine  the  electromotive  force  is 
generated,  as  was  shown  in  chapter  ii,  by  moving  a  wire 
near  a  magnet,  or  through  a  magnetic  field,  and  depends 
for  its  value  on  the  amount  of  magnetism  passed  through 
by  the  wire  per  second,  or,  as  it  is  generally  stated,  on  the 
number  of  lines  of  force  cut  per  second,  or  on  the  rate  of 
cutting  lines  of  force.  If  the  number  of  lines  of  force 
could  be  determined,  the  electromotive  force  in  volts  could 

(30) 


Generation  of  Electromotive  Force  in  Dynamos.  31 

be  calculated,  one  volt  being  generated  for  every  100,000,000 
lines  of  force  cut  per  second. 

In  machines  this  electromotive  force  depends  on  the 
speed  with  which  the  wire  passes  through  the  field,  and  on 
the  amount  of  magnetism  passed  through  by  that  wire;  it 
may  therefore  be  increased  by  increasing  either  of  these. 
The  amount  of  magnetism  passed  through  by  the  wire  may 
be  increased  by  one  or  more  of  the  following  different 
ways  : 

First.  By  increasing  the  intensity  of  the  magnetic  field, 
that  is,  the  magnetic  density  or  number  of  lines  of  force 
per  square  inch. 

Second.  By  increasing  the  size  of  the  field  of  the  same 
intensity  as  before,  or  in  other  words,  by  increasing  the 
total  number  of  lines  of  force  without  increasing  their 
density. 

Third.  By  increasing  the  number  of  armature  windings, 
or  in  other  words,  by  making  successive  parts  of  the  same 
continuous  wire  pass  simultaneously  through  the  same 
field,  by  winding  it  in  the  form  of  a  coil,  and  moving  this 
coil  through  the  field,  thus  generating  an  electromotive 
force  in  each  single  turn  or  winding  of  the  coil.  As  these 
turns  or  parts  of  turns  are,  from  the  nature  of  the  contin- 
uous winding  of  a  coil,  all  connected  in  series  with  one 
another,  it  follows  that  all  the  small  electromotive  forces 
which  are  generated  in  each  of  the  turns,  are  also  connected 
in  series  with  one  another,  thus  summing  all  the  small 
potentials  into  one  large  one. 

Fourth.  By  passing  the  same  wire  in  opposite  directions 
through  two  fields  of  opposite  polarities;  for  instance,  by 
causing  a  coil  of  one  turn  to  revolve  between  a  north  and 
a  south  pole,  as,  for  instance,  the  coil  6-5-4-3  in  figure  7,  in 
which  the  motions  of  the  parts  6-5  and  4-3  are  in  opposite 
directions,  and  the  magnetic  poles  near  which  they  are 
have  opposite  polarity.  From  the  laws  of  induction,  there- 
fore, the  currents  in  these  two  parts  will  tend  to  flow  in 
opposite  directions,  which,  as  will  be  seen  by  following  the 


32  Principles  of  Dynamo-Electric  Machines. 

current,  is  equivalent  to  inducing  a  current  in  the  same 
direction  around  the  coil,  or  in  other  words,  the  two  elec- 
tromotive forces  generated  in  the  two  parts,  6-5  and  4-3, 
will  be  added,  giving  double  that  induced  by  each  pole. 

The  electromotive  force  of  a  machine  may  therefore  be 
made  any  desired  number  of  volts,  by  simply  increasing 
any  one  or  all  of  the  following  :  the  speed,  the  intensity  of 
the  magnetism,  the  size  of  the  field  passed  through  in  the 
same  time,  the  number  of  times  that  different  parts  of  the 
same  wire  are  wound  so  as  to  pass  simultaneously  through 
the  same  field,  that  is,  the  number  of  armature  windings, 
and  lastly,  the  number  of  fields  passed  through  by  the 
same  wire.  Any  desired  potential,  however  large,  could 
be  generated  at  even  a  very  low  speed,  if  it  were  possible 
to  increase  the  magnetism  and  the  number  of  armature 
windings  to  the  proper  amount.  The  weak  magnetic  field 
of  the  earth  might  be  used  to  generate  a  very  high  elec- 
tromotive force  if  it  were  possible  to  increase  the  speed  of 
an  armature  or  the  number  of  its  windings,  or  both,  to  the 
required  amounts.  It  is  evident,  however,  that  such 
machines,  although  giving  the  required  electromotive  force, 
would  not  be  practical,  on  account  of  their  abnormal  pro- 
portions. As  there  are  so  many  different  combinations  of 
the  methods  just  given,  by,  means  of  which  the  same  elec- 
tromotive .  force  may  be  generated,  and  as  some  of  these 
will  be  more  practical  than  others,  or  less  expensive  in 
their  application,  it  follows  that  in  most  cases  there  will 
be  one  combination  which  is  the  cheapest  to  construct  and 
the  most  efficient  under  the  given  circumstances,  and  this 
proportion  of  parts  it  is  the  object  of  the  electrical  engineer 
to  determine. 

The  following  remarks  on  the  different  methods  of  in- 
creasing the  electromotive  force,  besides  those  already 
given  in  connection  with  magnetism,  and  those  which  will 
follow  in  discussing  the  details  of  construction  of  magnets 
and  armatures,  may  be  of  assistance  in  determining  the 
best  combination  of  parts  under  different  circumstances. 


Generation  of  Electromotive  Force  in  Dynamos.  33 

The  speed  of  the  wire  may  be  increased  by  increasing 
either  the  number  of  revolutions  or  the  diameter  of  the 
armature,  that  is,  the  distance  of  the  wire  from  the  axis 
of  rotation.  The  general  rule  in  regard  to  speed  is  to 
make  both  the  number  of  revolutions  and  the  diameter  as 
large  as  other  considerations  will  permit,  for  it  is  evident 
from  what  was  said  above,  that  the  higher  the  speed  the 
less  the  magnetism  or  the  number  of  windings  on  the  arma- 
ture, and,  therefore,  the  smaller  the  machine  for  generating 
the  same  electrical  energy.  The  limiting  conditions  for 
the  speed  are  purely  mechanical,  and  will  be  discussed 
under  details  of  construction. 

The  intensity  of  an  electro-magnet,  that  is,  the  number 
of  lines  of  force  per  square  inch,  may  be  increased  to  al- 
most any  amount ;  but,  as  iron  becomes  saturated,  the 
economical  limit  to  the  intensity  is  soon  reached.  If  a 
current  is  passed  through  a  solenoid,  that  is,  an  electro- 
magnet without  an  iron  core,  it  will  be  found  that  the 
magnetism  of  that  coil  increases  slowly  in  proportion  to 
the  current,  and  it  may  be  increased  to  any  desired 
amount  by  simply  increasing  the  current.  If  an  iron  core 
be  inserted,  and  the  same  series  of  experiments  made,  it 
will  be  found  that  the  amount  of  magnetism  for  the  same 
current  is  very  much  greater  than  it  was  before,  and  that 
it  also  increases  approximately  proportionately  to  the  cur- 
rent. But  a  limit  is  soon  reached  when  the  iron  ceases  to 
add  to  the  increasing  amount  of  magnetism,  and  it  is  then 
said  to  be  saturated.  After  this  point  is  reached  the  mag- 
netism will  still  increase  with  the  current,  but  only  slowly, 
as  in  the  first  case  of  a  coil  without  a  core.  This  point  of 
saturation  is  the  degree  of  magnetization  at  which  all 
machines  should  be  used  in  practice,  in  order  to  give  the 
greatest  amount  of  magnetism  for  the  smallest  expendi- 
ture of  iron,  wire  and  current.  If  a  less  degree  of  mag- 
netization is  used  the  magnets  are  unnecessarily  large,  and 
the  amount  of  copper  wire  therefore  unnecessarily  great. 
If  the  iron  is  over-saturated,  then  the  amount  of  magnetism 


34  Principles  of  Dynamo- Electric  Machines. 

developed  by  a  certain  amount  of  current  is  not  as 
great  as  it  might  be,  and  therefore  the  proportions  are  not 
the  most  economical. 

From  this  the  general  rules  follow :  Diminish  the 
amount  of  iron  so  that  the  magnets  may  be  just  at  the 
point  of  saturation  for  the  required  amount  of  magnetism. 
Also,  proportion  the  cross  section  of  the  iron  at  different 
parts  of  the  magnetic  circuit,  so  that  it  may  all  be  satur- 
ated at  once.  This  will  be  further  discussed  under  the 
subject  of  magnets. 

The  size  of  the  magnets  should  evidently  be  as  small  as 
the  required  amount  of  magnetism  and  the  saturation 
limit  will  permit,  for  reasons  just  given.  This  must  not 
be  confounded  with  the  practical  rule  which  will  be  ex- 
plained hereafter,  namely,  that  the  amount  of  magnetism 
should  be  as  great  as  practicable,  which  simply  means  that 
the  amount  of  magnetism,  as  distinguished  from  the  num- 
ber of  armature  windings,  should  be  made  great,  but  that 
for  this  given  amount  of  magnetism  the  size  of  the  mag- 
net should  not  be  larger  than  the  saturation  limit  will 
necessitate,  as  the  machine  will  otherwise  become  unneces- 
sarily large  and  bulky,  and  the  amount  of  wire  on  it  will 
be  greater  than  it  need  be. 

The  magnets  should  be  of  the  construction  best  adapted 
to  collect  and  concentrate  all  the  lines  of  force  generated 
by  the  magnet  coils,  into  the  space  through  which  the 
moving  wire  passes,  as  it  is  evident  that  only  those  lines  of 
force  which  pass  through  this  space  will  be  rendered  use- 
ful. All  the  others  are  wasted.  This  subject  of  size  and 
shape  of  magnets  will  be  further  discussed  under  the  sub- 
ject of  details  of  construction  of  magnets. 

The  third  method  of  increasing  the  electromotive  force, 
namely,  by  causing  the  same  continuous  wire  to  pass  re- 
peatedly through  the  same  field,  is  best  illustrated  by  the 
ordinary  well-known  Gramme  armature.  An  iron  ring, 
figure  6,  is  wound  spirally  with  a  continuous  wire,  the 
end  of  which  is  connected  to  the  beginning,  so  as  to  make 


Generation  of  Electromotive  Force  in  Dynamos. 


35 


it  an  endless  coil  closed  in  itself.  This  armature  is  then 
revolved  between  a  north  and  south  pole,  as  shown  in  the 
figure.  If  the  direction  of  rotation  and  the  polarity  of  the 
magnet  poles  are  as  shown,  then  the  currents  induced  in  all 
the  portions  of  the  wire,  which  are  between  the  iron  ring 


and  the  north  pole,  will  tend  to  flow  toward  the  observer, 
according  to  the  rules  of  induction,  while  in  the  wires 
under  the  influence  of  the  south  pole,  the  currents  will 
tend  to  flow  away  from  the  observer.  From  the  nature 
of  the  winding,  it  will  be  noticed  by  following  the  wire 
around  the  core,  that  the  active  portions  between  the  iron 
ring  and  the  pole  piece,  are  connected  in  series  with  one 
another  by  those  portions  of  the  wire  which  lie  in  the  in- 
side of  the  ring,  or,  in  other  words,  the  same  wire  is  wound 
so  that  it  returns  through  the  inside  of  the  ring,  and  passes 
repeatedly  through  the  same  field.  The  small  electromotive 
forces  induced  in  each  of  these  active  parts  as  they  pass 
simultaneously  through  the  field,  are  therefore  added  in 


36  Principles  of  Dynamo- Electric  Machines. 

series,  and  at  the  two  points  a  and  b  of  that  wire,  where 
it  enters  and  leaves  the  north  magnetic  field,  the  total 
electromotive  force  will  be  the  sum  of  all  the  small  elec- 
tromotive forces  induced  in  each  turn.  The  potential 
which  it  is  possible,  in  practice,  to  generate  in  a  single 
wire  in  passing  through  the  field  once,  is  exceedingly 
small,  being  in  the  more  common  machines  about  a  volt,  and 
in  some  exceptional  (and  impractical)  cases  a  few  volts.  It 
is  therefore,  necessary,  in  all  machines  to  cause  successive 
parts  of  the  same  wire  to  pass  simultaneously  through  the 
same  field,  in  order  not  to  have  the  magnets  too  large  and 
costly,  nor  the  speed  too  great  for  safe  running  ;  in  other 
words,  the  armature  must  contain  numerous  windings. 

As  the  small  potentials  are,  from  the  nature  of  the  con- 
tinuous winding,  all  connected  in  series,  the  well-known 
law  becomes  evident,  namely,  that  the  electromotive  force 
of  such  an  armature  is  approximately  proportional  to  the 
number  of  turns  or  windings  of  the  wire  around  the  core. 
Or  in  other  words,  to  increase  the  electromotive  force  it  is 
only  necessary  to  increase  the  number  of  windings  in  the 
same  proportion,  provided  this  can  be  done  without  chang- 
ing any  other  parts  on  which  the  electromotive  force  de- 
pends. This  electromotive  force  must  not  be  confounded 
with  the  available  difference  of  potential  at  the  poles  of 
the  machine  when  running,  as  mentioned  in  chapter  iii,  for 
the  latter  will  not  increase  in  proportion  to  the  windings, 
as  it  depends  on  the  current  which  is  flowing  and  on  the 
size  of  the  wire  ;  it  will  in  all  cases  be  less  than  the  elec- 
tromotive force. 

Referring  again  to  figure  6,  we  notice  that  in  that  por- 
tion a  b  of  the  continuous  wire  around  the  core  which  is 
under  the  influence  of  the  north  pole  piece,  the  summing 
up  of  the  small  potentials  causes  the  total  to  act  at  a  and 
b,  making  a  positive  pole  at  a  and  a  negative  at  b.  In  ex- 
amining that  portion  of  the  same  continuous  wire  which  is 
in  the  other  field,  the  same  is  the  case,  with  this  difference 
only,  that  the  direction  of  the  inducted  current  will  be  the 


Generation  of  Electromotive  Force  in  Dynamos.  37 

reverse,  thereby  causing  a  positive  pole  to  be  developed  at 
c  and  a  negative  at  d,  which  can  readily  be  proved  by  ap- 
plying the  rules  of  induction  given  in  chapter  iii.  The 
two  currents,  therefore,  oppose  each  other  at  the  two  points 
of  the  wire  which  are  not  under  the  influence  of  the  mag- 
netic fields.  These  are  the  points  at  which  the  current 
must  be  led  off  by  the  usual  method  of  applying  brushes 
at  these  places,  as  they  remain  fixed  in  position  under  the 
same  circumstances,  while  the  wire  moves.  This  might  be 
done  by  making  the  wire  bare  at  those  parts  which  have 
to  come  in  contact  with  the  brushes,  and  then  applying  the 
brushes  directly  to  the  outside  of  the  armature.  But  as 
this  is  not  practicable,  for  numerous  reasons,  the  contin- 
uous wire  is  connected  at  regular  and  frequent  intervals  to 
the  thick,  insulated  bars  of  the  collector,  or  so-called  com- 
mutator, on  which  the  brushes  are  made  to  bear. 

The  function  of  this  collector  in  a  Gramme  ring  armature, 
is,  therefore,  merely  a  mechanical  one,  to  prevent  the 
wearing  off  of  the  wires,  and  to  present  a  broad  contact 
for  the  brushes.  It  is  therefore  not  really  a  "  commuta- 
tor," its  function  not  being  electrical,  as  the  brushes  might 
just  as  well  be  applied  to  the  outside  of  the  armature  di- 
rectly on  the  wires  if  they  are  bare,  were  it  not  for  the 
mechanical  objections. 

It  is  evident  from  the  figure,  that  in  this  form  of  wind- 
ing, the  two  halves  of  the  continuous  winding  on  the  arma- 
ture are  necessarily  connected  in  multiple  arc,  and  there- 
fore each  half  supplies  the  same  electromotive  force,  but 
only  half  the  current  of  the  machine. 

From  the  above  remarks  the  following  practical  rules 
will  be  readily  understood.  As  the  two  halves  of  the  con- 
tinuous wire  are  from  the  nature  of  the  winding,  connected 
in  multiple  arc,  it  is  very  important  that  the  electromotive 
forces  of  the  two  halves  should  be  equal,  for  if  they  are 
not,  the  difference  between  the  two  will,  under  certain  cir- 
cumstances, cause  a  reverse  current  to  circulate  in  the 
armature  wire,  and  as  this  current  does  not  appear  in  the 


38  Principles  of  Dynamo- Electric  Machines. 

external  circuit,  it  is  lost  energy  which  might  easily  have 
been  saved.  The  brushes  should,  therefore,  be  directly  op- 
posite to  each  other,  and  the  magnetic  fields  should  be  per- 
fectly balanced,  that  is,  both  fields  should  contain  an  equal 
number  of  lines  of  force. 

As  the  two  halves  of  the  continuous  wire  are  in  multiple 
arc,  it  follows  that  the  real  resistance  of  the  whole  armature 
from  brush  to  brush,  is  one-half  the  resistance  of  one-half  of 
the  wire,  which  is  evidently  equivalent  to  one-quarter  of  the 
resistance  of  the  whole  wire.  As  the  whole  current  of  the 
machine  divides  into  two  parts  in  the  armature,  the  wire 
need  be  calculated  for  only  half  the  current  of  the  machine. 
The  two  halves  being  in  multiple  arc,  the  total  electromotive 
force  is  evidently  only  half  that  which  the  same  amount 
of  wire,  windings,  speed  and  field,  could  produce.  By 
some  writers  this  is  considered  to  be  a  grave  fault  of  that 
form  of  armature ;  a  little  thought  will,  however,  show 
that  it  is  not  such  a  serious  fault,  as  the  current  thereby  is 
twice  as  great  as  the  same  size  wire  could  stand  alone, 
and  therefore  the  electrical  energy  developed,  or  the  cur- 
rent multiplied  by  the  electromotive  force,  is  the  same. 
If  the  two  halves  could  be  connected  in  series,  the  wire 
would  evidently  have  to  be  about  twice  as  large  in  cross 
section,  and  therefore  the  whole  machine  would  become 
larger.  The  only  practical  loss  due  to  this  so-called  fault, 
is,  that  slightly  more  space  is  occupied  by  the  insulation 
on  two  thin  wires  in  multiple  arc,  than  would  be  required 
for  one  thicker  wire.  A  device  has  been  patented  for 
connecting  the  two  halves  of  a  Gramme  ring  armature  in 
series,  but  as  the  small  advantage  gained  is  obtained  at  the 
cost  of  simplicity,  it  is  no  improvement  on  the  old  form  of 
Gramme  ring. 

Referring  again  to  figure  6,  it  will  be  seen  that  in  order 
to  permit  the  wire  to  be  wound  so  that  successive  portions 
pass  simultaneously  through  the  same  field,  a  portion  of 
every  winding  must  return  from  one  end  of  the  armature 
to  the  other  in  a  space  in  which  no  induction  takes  place, 


Generation  of  Electromotive  Force  in  bynamos.  3d 

that  is  in  the  space  enclosed  by  the  iron  ring.  As  the  lines 
of  force  which  enter  the  iron  of  the  ring  are  led  through 
the  core  to  the  opposite  field,  it  is  evident  that  none  will 
pass  through  the  space  enclosed  by  the  ring,  which  has 
been  repeatedly  demonstrated  with  iron  filings.  The  wire 
in  the  inside  of  the  ring  is  therefore  inactive,  and  may  be 
considered  dead  resistance,  because  there  is  no  electromo- 
tive force  induced  in  it.  It  serves  the  purpose  merely  of 
connecting  the  two  ends  of  the  active  portions  which  pass 
through  the  fields.  For  the  same  reason  the  wires  at  the 
ends  of  the  ring  are  also  dead,  from  which  it  follows  that 
in  many  of  the  Gramme  ring  armatures,  the  dead  wire 
forms  from  60  to  70  per  cent,  of  the  whole  armature  re- 
sistance. According  to  the  old  theory  of  induction,  this 
wire  is  not  dead,  and  the  question  is,  therefore,  still  a  dis- 
puted one  ;  we  believe,  however,  that  the  best  authorities 
agree  in  saying  that  it  is  dead.  (  A  very  simple  experiment 
could  readily  be  made  to  settle  this  important  question.1 ) 

To  overcome  this  alleged  fault,  the  ingenious  method 
was  devised,  of  making  the  return  wire  of  each  winding 
pass  through  the  opposite  field,  instead  of  through  the  in- 
side of  the  ring,  thus  rendering  it  active  in  generating 
electromotive  force.  This,  we  termed  above,  the  fourth 
method  of  increasing  the  electromotive  force.  Instead  of 
making  the  iron  in  the  form  of  a  ring,  it  may  then  be  made 
a  solid  cylinder,  around  the  outside  of  which  the  wire  is 
wound.  It  then  becomes  the  well-known  cylinder  arma- 
ture shown  in  figure  7.  In  this  case  the  continuous  wire, 
as  before,  passes  repeatedly  through  the  same  field,  but  in- 
stead of  being  partially  in  a  space  in  which  there  is  no 
magnetism,  half  of  it  lies  on  the  other  side  of  the  arma- 
ture, in  the  other  field,  in  which  induction  also  takes  place. 
In  applying  the  rules  of  induction  it  will  be  seen  that  the 
current  is  induced  in  opposite  directions  in  the  same  wire, 
in  the  two  fields,  but  upon  following  these  two  directions 
along  the  same  wire,  it  will  be  seen  that  they  are  really 
the  same  direction  with  reference  to  the  continuous  wire 

i  See  Appendix  II. 


40 


Principles  of  Dynamo-Electric  Machines. 


itself,  and  that,  therefore,  the  two  electromotive  forces  in- 
duced in  the  same  wire  in  passing  through  two  opposite 
fields,  are  added  in  series.  This  will  be  seen  in  figure  7, 
by  following  the  continuous  wire  around  the  armature, 
starting  at  1,  and  thence  in  regular  order  to  8.  From  9  to 
16  the  same  will  be  found  to  be  the  case,  only  that  the  cur- 
rent in  this  half  is  opposite  to  that  in  the  first  half,  and 
therefore  precisely  similar  in  this  respect  to  the  two  halves 
of  the  wire  in  the  Gramme  ring.  All  that  was  said  about 
the  latter,  as  regards  the  two  halves  being  connected  in 
multiple  arc,  also  applies  to  cylinder  armatures.  The  only 
wire  which  can  be  termed  dead  resistance  in  the  cylinder 


FIGURE  7. 

armature,  is  that  at  its  ends.  By  making  the  armature 
long  in  comparison  to  its  diameter,  this  dead  wire  may  be 
reduced  to  a  small  proportion  of  the  whole.  In  a  recent 
invention  even  this  part  of  the  armature  wire  has  been 
rendered  active. 

The  winding  of  the  cylinder  armature,  which  is  often 
represented  as  being  very  complicated,  is  in  general  very 
simple.  It  may  be  considered  to  be  an  ordinary  Gramme 
ring  winding,  in  which  the  wire,  instead  of  being  wound 
alternately  on  the  outside  and  inside  of  the  ring,  is  wouncj 
on  opposite  sides  of  the  core,  in  which  case,  the  iron  ring 
may  be  replaced  by  a  solid  cylinder,  as  there  is  no  longer 


Generation  of  Electromotive  Force  in  Dynamos.  41 

any  object  in  having  the  hollow  space  in  the  inside.  The 
cylinder  winding  may  also  be  said  to  be  similar  to  the 
winding  of  a  ball  of  twine,  the  wire  being  wound  around 
the  outside  while  the  cylinder  is  being  turned  around 
slowly  on  its  axis.  In  this  case,  as  well  as  in  the  Gramme 
ring,  the  end  of  the  wire  after  completing  the  winding,  is 
connected  to  the  beginning. 

These  two  types  of  armature  winding  are  given  here 
merely  as  illustrations  of  methods  for  increasing  the  elec- 
tromotive force  in  machines,  by  induction  in  successive 
parts  of  the  same  wire  simultaneously,  in  the  same  mag- 
netic field  or  pair  of  fields.  They  will  be  further  discussed 
under  the  head  of  details  of  construction. 


CHAPTER   V. 

Armatures. 

THE  foregoing  chapter  having  completed  the  short  re- 
view of  the  general  and  leading  principles  involved  in  the 
generation  of  electrical  energy  by  machines,  the  remaining 
chapters  will  be  devoted  to  a  discussion  of  the  practical 
application  of  those  principles  to  the  construction  of  ma- 
chines, together  with  practical  rules  and  hints  in  regard  to 
the  proportioning  of  parts,  both  of  armatures  and  of  the 
field  magnets.  As  almost  all  of  the  machines  built  in  this 
country  use  either  the  Gramme  or  cylinder  armatures,  or 
variations  of  them,  such  as  the  Thomson-Houston  or 
Brush,  the  discussion  on  armatures  will  be  limited  to 
these  general  types.  Both  of  these  forms  being  very  simi- 
lar in  many  respects,  and  the  first  being  the  simplest  in 
form,  the  first  part  of  the  present  chapter  will  be  limited 
to  the  Gramme  type;  the  subject  of  cylinder  armatures 
will  be  confined  merely  to  the  differences  between  it  and 
the  Gramme. 

In  order  to  appreciate  more  fully  the  reasons  for  many 
of  the  practical  rules  for  constructing  armatures,  let  us 
examine  in  detail  what  takes  place  in  a  Gramme  armature 
while  running.  In  figure  8  is  shown  a  diagrammatic  repre- 
sentation  of  a  simple  Gramme  armature  with  its  collector, 
brushes  and  pole  pieces,  the  directions  of  the  currents  and 
lines  of  force  being  represented  by  arrows,  as  they  would 
be  while  the  machine  is  running  in  the  direction  indicated, 
the  polarity  of  the  pole  pieces  being  as  represented. 

The  direction  of  the  currents  may  be  determined  by 
the  practical  rule  given  in  chapter  iii.  By  grasping  the 
north  pole  piece  in  the  right  hand,  the  middle  finger, 
which  points  down,  will  indicate  the  direction  of  the  current 

(42) 


Armatures. 


43 


if  the  wires  between  the  armature  core  and  the  north 
pole  piece  were  moving  in  the  direction  in  which  the 
thumb  points,  which  in  this  case  would  be  like  the  hands 
of  a  watch.  As  it  turns  in  the  opposite  direction,  how- 
ever, the  currents  will  also  be  in  the  opposite  direction, 
which  in  this  case  will  be  from  below  up,  thus  giving  the 
direction  around  the  end  of  the  armature  as  indicated.  A 
similar  application  of  the  rule  will  show  the  direction  of 


the  currents  at  the  south  pole  piece  to  be  as  indicated 
there,  namely  opposite  to  those  at  the  north  pole  in  the 
cylindrical  portion  of  the  armature,  but  in  the  same  direc- 
tion at  the  end  of  the  armature,  as  will  be  seen  in  the 
figure. 

From  this  the  practical  rule  of  thumb  follows,  for  de- 
termining the  direction  of  the  currents  in  all  types  of 
Gramme  as  well  as  cylinder  armatures,  viz. :  in  looking  at 


44  Principles  of  Dynamo-Electric  Machines. 

the  commutator  end  of  the  armature,  if  the  rotation  is 
opposite  to  that  of  the  hands  of  a  toatch,  the  currents  at 
that  end  will  have  the  same  direction  as  the  lines  of  force, 
namely  from  the  north  to  the  south  pole  pieces.  In  fol- 
lowing one  of  these  currents  to  the  brushes  their  polarity 
may  be  readily  determined.  This  polarity  of  the  brushes 
will  evidently  be  different,  depending  on  whether  the  gen- 
eral direction  of  the  windings  of  the  armature  is  a  right 
hand  or  a  left  hand  spiral,  which  can  readily  be  determined 
by  following  a  wire  from  one  commutator  strip  to  the 
next.  If  a  correct  drawing  has  been  made  of  a  machine, 
on  whicli  the  polarity  of  the  pole  pieces,  the  direction  of 
rotation,  and  the  general  direction  of  the  spiral  wind- 
ing in  the  armature,  are  indicated,  the  polarity  of 
the  brushes  and  the  binding  posts,  can  evidently  be 
determined  before  the  machine  is  built,  provided  the 
machine  is  constructed  as  designed. 

All  the  small  electromotive  forces  induced  in  each  of 
the  windings  of  the  wire  around  the  armature  core,  will, 
from  the  nature  of  the  continuous  winding,  be  added  in 
series,  and  therefore  at  the  brushes  the  total  electromotive 
force  will  be  the  sum  of  all  the  smaller  ones  on  one-half  of 
the  armature,  as  explained  in  the  last  chapter.  Supposing 
the  electromotive  force  induced  in  each  winding  to  be  the 
same,  the  practical  rule  follows  that  the  electromotive 
force  of  a  machine  is  proportional  to  the  number  of 
windings  on  the  armature  ;  and  it  is  equal  to  that  induced 
in  one  winding  multiplied  by  the  total  number  of  wind- 
ings divided  by  two,  as  the  two-halves  of  the  armature 
windings  are  in  multiple  arc. 

The  pole  pieces  will  induce  a  north  and  a  south  pole  in 
those  parts  of  the  iron  core  of  the  armature  directly  ad- 
joining them,  as  shown  at  the  letters  s  and  N  on  the  core. 
If  this  were  the  only  magnetization  of  the  core  the  lines 
of  force  would  take  the  shortest  direction  through  the 
armature  core,  and  the  line  of  the  brushes  a  b,  or  diameter 
of  commutation,  as  it  is  called,  would  be  perpendicular  to 


Armatures.  45 

the  magnetic  axis  of  the  pole  pieces.  In  examining  the 
magnetic  effect  of  the  currents  in  the  armature  wire,  it 
will  be  seen  that  these  currents  also  magnetize  the  core, 
in  each  half  of  which  they  tend  to  make  two  opposite 
poles,  thus  making  a  separate  magnet  of  each  half  of  the 
core,  with  north  poles  at  nn  and  south  poles  at  ss,  as  will 
be  seen  if  the  core  is  imagined  to  be  cut  into  two  parts 
through  the  line  of  the  brushes  a  b,  and  the  magnetic  po- 
larity determined  at  each  end.  These  two  independent 
magnetizations,  one  by  the  pole  pieces  and  the  other  by 
the  armature  current,  will  tend  to  oppose  each  other  to  a 
certain  extent,  and  will  produce  resultant  poles  at  N'  s'. 
These  will  be  the  real  effective  poles  in  the  induction  and 
the  lines  of  force  will,  therefore,  be  crowded  to  one  corner 
of  each  pole  piece,  as  shown  in  dotted  lines,  some  of  which 
are  shown  as  they  pass  through  the  armature  core.  As 
the  line  of  the  brushes,  a  b  (that  is,  the  line  through  the 
two  points  where  the  currents  induced  in  the  two 
halves  of  the  armature,  meet),  will  be  perpendicular,  or 
nearly  so,  to  this  line  of  magnetization,  it  follows  that  the 
brushes  will  have  to  be  shifted  in  the  direction  of  rotation. 
This  shifting  of  the  brushes  is  often  erroneously  attrib- 
uted entirely  to  the  so-called  magnetic  lag,  which  is  the 
tendency  of  the  iron  core  to  retard  its  magnetization  when 
entering  the  field,  and  to  retard  its  demagnetization  when 
leaving  it.  Although  this  is  the  case  to  a  slight  extent,  it 
is,  in  a  soft  iron,  laminated  armature,  very  small  as  com- 
pared to  the  transverse  magnetization  by  the  armature 
current.  It  is  very  easy  to  determine  how  much  of  this 
displacement  of  the  brushes  is  due  to  the  magnetic  lag, 
by  finding  the  position  of  the  brushes  at  which  the  great- 
est potential  exists  when  the  machine  is  running  on  open 
circuit,  that  is,  without  a  current  in  the  armature,  in  which 
case  the  magnets  must  be  excited  by  another  machine. 
This  amount  of  displacement  is  then  evidently  due  entirely 
to  the  magnetic  lag,  provided  the  armature  is  perfectly 
balanced  electrically. 


46  Principles  of  Dynamo- Electric  Machines. 

The  shifting  of  the  line  of  the  brushes,  when  the  arma- 
ture is  generating  current,  will  be  greater  in  proportion 
to  the  magnetization  due  to  the  armature  coils,  that  is,  in 
proportion  to  the  number  of  windings  on  the  armature; 
as  its  effect  is  very  objectionable,  because  it  practically 
makes  the  effective  pole  pieces  narrow,  causes  bad  spark- 
ing, etc.,  the  important  rule  becomes  evident,  viz.:  make 
the  number  of  windings  on  the  armature  as  small  as  pos- 
sible, and  therefore  the  other  two  factors  on  which  the 
electromotive  force  depends,  namely  the  speed  and  the 
field,  should  be  made  correspondingly  great.  There  are, 
also,  other  reasons  for  this  same  important  rule,  which  will 
be  mentioned  below.  It  is  one  of  the  most  important 
rules  in  the  construction  of  a  well  proportioned  machine. 
In  several  cases  in  the  writer's  experience  the  capacity  of 
a  machine,  in  lamps,  was  doubled  without  any  increase  of 
size  of  the  frame,  by  a  proper  application  of  this  rule. 

If  one  of  the  brushes  were  to  leave  a  commutator  strip 
before  it  touches  the  next,  the  circuit  would  be  momen- 
tarily broken  at  that  instant,  which  would  result  in  form- 
ing a  succession  of  small  arcs.  This  is  generally  termed 
sparking,  and  is  very  injurious,  as  it  burns  off  the  brushes 
and  the  commutator  bars.  It  is,  therefore,  absolutely  es- 
sential that  a  brush  should  bridge  over  the  insulation  be- 
tween two  commutator  bars  and  touch  one  bar  before  it 
leaves  the  other,  as  shown  in  figure  8. 

From  the  nature  of  an  armature  winding,  two  neighbor- 
ing commutator  strips  represent  the  two  ends  of  one  of  the 
coils  of  the  armature,  and  it  therefore  follows  that  while 
a  brush  touches  two  strips,  that  coil  which  terminates  at 
those  two  strips,  is  short  circuited  through  the  brushes,  as 
shown  in  the  two  coils  marked  o  o.  This  current  repre- 
sents a  certain  amount  of  loss,  as  it  does  not  appear  in  the 
external  circuit.  When  the  brush  leaves  a  commutator 
strip,  after  having  short  circuited  this  coil,  this  local  cur- 
rent is  broken  at  the  terminal  of  the  brush,  and  therefore 
causes  sparking.  This  is,  perhaps,  one  of  the  chief  among 


Armatures.  47 

the  many  causes  of  sparking,  and  should  therefore  not  be 
overlooked.  As  this  short  circuiting  cannot  be  avoided 
in  the  ordinary  form  of  commutator,  the  only  thing  that 
can  be  done  is  to  make  the  current  in  that  coil  as  small  as 
possible.  It  may  be  argued  that  the  induced  electromo- 
tive force  in  this  coil  is  always  small,  but  it  must  also  be 
remembered  that  it  is  short  circuited  by  a  very  low  resis- 
tance and  that  the  current  may  therefore  be  very  great. 
For  instance,  if  the  induced  electromotive  force  in  that 
coil  be  only  .1  volt  and  the  resistance  of  that  one  coil 
.001  ohm,  the  local  current  would  evidently  be  100  amperes 
by  Ohm's  law,  and  when  a  current  of  100  amperes  is  sud- 
denly broken  a  spark  will  be  produced,  especially  as  the 
current  flows  through  a  coil  which  has  self  induction,  the 
action  of  which  is  to  tend  to  prolong  the  duration  of  the 
spark  or  arc,  by  suddenly  increasing  its  potential. 

The  only  way  to  reduce  this  objectionable  current  is  to 
make  the  induction  in  that  coil  as  small  as  possible  and  to 
decrease  its  self  induction.  The  coil  should,  therefore,  al- 
ways lie  near  the  neutral  point  in  the  field,  and  should  be 
in  the  weakest  part  of  it;  it  should,  moreover,  move  in  the 
direction  of  the  lines  of  force  so  that  there  is  little  or  no 
induction  in  it. 

The  ends  of  the  opposite  pole  pieces,  p  p,  nearest  each 
other  should,  therefore,  be  as  far  apart  as  practicable,  so 
that  as  few  lines  of  force  as  possible  pass  directly  from  one 
to  the  other  through  these  two  short  circuited  coils.  A 
good  rule  is  to  make  this  distance,  P  P,  at  least  eight  times 
the  distance  between  the  iron  of  the  pole  pieces  and  the 
iron  armature  core.  It  should  even  be  more  if  practicable, 
but  not  less.  From  this  it  also  follows  that  the  distance 
from  the  pole  piece  to  the  armature  core,  or  in  other  words 
the  depth  of  the  windings  on  the  armature,  should  be  as 
small  as  practicable.  To  keep  the  short  circuited  coil  in 
the  weakest  part  of  the  field  the  diameter  of  commutator, 
or  line  of  brushes,  should  be  shifted  as  little  as  possible 
from  the  position  normal  to  the  line  joining  the  pole  pieces, 


48  Principles  of  Dynamo-Electric  Machines. 

in  the  case  of  a  Gramme  armature.  This  is  another  rea- 
son for  making  the  armature  windings  as  few  as  possible, 
as  stated  above.  The  field  should  be  balanced,  that  is, 
it  should  be  symmetrical  both  magnetically  and  in  the  out- 
line of  the  iron  parts,  in  order  that  not  only  the  brushes 
but  also  the  short  circuited  coils  should  be  exactly  diamet- 
rically opposite.  The  brushes  should  not  short  circuit 
more  than  one  coil,  and  should  therefore  not  touch  more 
than  two  strips  at  any  time  ;  neither  should  there  be  two 
brushes  on  each  side,  the  distance  between  which  is  greater 
than  the  width  of  a  commutator  strip,  as  they  would  then 
short  circuit  more  than  one  coil.  Finally,  to  decrease  the 
self  induction  of  these  short  circuited  coils,  the  number  of 
windings  per  coil  should  be  as  small  as  possible  and 
consequently  the  important  rule  that  the  number  of  coils  or 
number  of  commutator  strips  should  be  as  great  as  possible 
to  diminish  the  number  of  windings  per  coil.  As  the  self 
induction  increases  with  the  square  of  the  number  of  wind- 
ings in  the  coil,  halving  this  number  by  doubling  the  num- 
ber of  coils  or  commutator  strips  will  diminish  the  self 
induction  in  the  short  circuited  coil  to  one-fourth  of  what 
it  was  before. 

As  the  direction  in  which  the  currents  tend  to  be  in- 
duced in  all  the  coils  of  the  armature  depends  only  on  the 
polarity  of  the  pole  pieces  and  the  direction  of  rotation, 
it  is  independent  of  the  position  of  the  brushes.  From 
this  it  will  be  seen  that  if  the  brushes  are  moved  to 
different  parts  of  the  circumferenee  of  the  commutator 
the  direction  in  which  the  current  will  have  to  flow  in 
some  of  the  coils  will  be  opposed  to  that  induced  in  them, 
and  therefore  the  electromotive  force  of  these  opposing 
coils  will  be  subtracted  from  the  rest  and  the  difference 
will  appear  at  the  brushes.  This  is  often  made  use  of 'to 
regulate  the  electromotive  force  of  a  machine.  It  is  simi- 
lar to  regulating  the  difference  of  potential  of  a  set  of  ac- 
cumulators, by  connecting  one  or  more  so  as  to  oppose  the 
others.  The  difference  in  the  amounts  of  energy  at  the 


Armatures.  49 

poles  is  therefore  not  wasted,  but  is  stored  up  again  in  the 
few  cells  which  oppose  the  others  ;  or  in  the  case  of  a 
dynamo  the  current  in  the  opposing  coils  tends  to  make  it 
a  motor.  This  method  of  regulating  is  therefore  in  that 
sense  not  uneconomical,  but  it  has  the  very  serious  objec- 
tion that  as  the  brushes  are  shifted  to  different  parts  of 
the  commutator,  the  coils  which  are  short  circuited  by 
them  are  no  longer  in  a  weak  neutral  field  but  have  con- 
siderable electromotive  force  generated  in  them,  and 
therefore  the  current  circulating  in  them  when  short  cir- 
cuited by  a  brush  will  be  very  great,  and  the  armature  will 
therefore  heat,  waste  energy,  and  spark  badly  at  the  com- 
mutator the  more  the  brushes  are  moved  from  their  proper 
position.  Unless  some  device  is  used  for  blowing  out  these 
sparks,  as  in  the  Thomson-Houston  machine,  such  regula- 
tion should  not  be  used  in  well  built  machines  except  with- 
in very  small  limits. 

When  two  brushes  are  used  on  each  side  of  the  commu- 
tator, both  leading  to  one  terminal,  the  coils  lying  between 
them  are  necessarily  short  circuited  by  them,  and  if  there 
is  the  slightest  induction  in  these  coils  there  will  be  a  local 
current  flowing  through  them,  which  heats  the  armature, 
wastes  energy  and  causes  bad  sparking. 

Another  cause  of  sparking  is  the  self  induction  of  the 
coils  in  the  armature.  This  is  similar  to  inertia  in  me- 
chanics, and  is  that  quality  of  the  coils  by  virtue  of  which 
they  resist  the  starting  or  stopping  of  a  current  through 
them.  It  acts  every  time  a  brush  makes,  or  breaks  contact 
with  the  next  commutator  strip,  as  this  starts  or  stops  a 
current  in  the  corresponding  coil.  As  this  self  induction 
is  proportional  to  the  square  of  the  number  of  windings  in 
the  coil,  the  same  important  rule  follows  that  the  number 
of  windings  on  the  armature  should  be  as  small  as  possible, 
and  for  a  given  number  of  such  windings,  each  coil  should 
have  as  few  as  possible  by  making  the  number  of  coils  or 
commutator  strips  correspondingly  great. 

It  will  be  seen  from  figure  8  that  the  wires   of  two 


50  Principles  of  Dynamo-Electric  Machines. 

neighboring  coils  never  have  a  greater  difference  of  poten- 
tial in  them  than  that  generated  in  one  or  two  coils.  For 
instance,  if  there  were  64  coils  and  the  machine  gave  64 
volts,  it  is  evident  that  each  coil  generated  on  the  average 
about  two  volts,  as  32  coils  are  connected  in  series.  Two 
volts,  or  at  most  four  volts,  is  the  greatest  potential  in  any 
two  neighboring  wires.  There  is,  therefore,  no  likelihood 
of  the  current  "jumping"  from  one  to  the  other,  which  is 
the  reason  why  the  Gramme  armature  is  especially  well 
adapted  for  high  potentials.  This  is  a  characteristic  and 
important  difference  between  it  and  the  cylinder  arma- 
ture, in  which  the  whole  difference  of  potential  can  exist 
between  two  neighboring  wires,  as  will  be  explained 
later.  In  all  armatures  it  is  important  to  insulate,  very 
carefully,  the  wires  from  the  core,  for  if  this  insulation 
should  be  crushed  or  should  rub  through  at  one  point,  the 
whole  electromotive  force  of  the  machine  will  act  at  an- 
other point  to  burst  through  the  insulation  and  cause  the 
current  to  "jump"  through  to  the  core,  thus  burning  out 
the  armature.  This  is  the  most  frequent  cause  of  arma- 
tures burning  out ;  an  armature  should  not  be  used  in 
which  any  one  point  of  the  wire  touches  the  iron  core. 
The  core  should  have  all  corners  and  edges  rounded  ;  it 
should  have  no  sharp  ridges,  or  burrs  produced  while  turn- 
ing it  in  the  lathe,  and  above  all  there  should  be  no  loose 
parts  which  may  rub  against  the  wire,  and  no  parts  which 
can  get  loose  when  the  armature  expands  on  being  heated. 
As  the  two  halves  of  the  armature  winding  are  in  multi- 
ple arc,  it  is  very  necessary  to  have  the  electromotive 
forces  induced  in  each  half  equal  to  each  other.  For  sup- 
pose there  was  a  difference  of  one  volt,  it  is  evident  that 
when  the  machine  is  running  on  open  circuit  there  will  be 
a  wasted  local  current  circulating  through  the  armature 
wire,  which  will  be  approximately  equal  to  one  volt  divided 
by  the  resistance  of  the  two  halves  of  the  armature  wire 
in  series.  If  this  were  .01  ohm,  this  local  current  circula- 
ting in  the  armature  would  be  almost  100  amperes,  the 


Armatures.  51 

self  induction  tending  to  reduce  it  somewhat.  This  local 
current  becomes  less  as  the  current  in  the  external  circuit 
increases.  It  is,  therefore,  very  important  to  have  the 
same  number  of  turns  in  all  the  armature  coils,  to  have 
them  all  symmetrically  situated  with  respect  to  the  field 
and  the  axis,  and  to  have  a  balanced  field. 

This  is  especially  important  in  a  cylindrical  armature, 
which  is  often  less  symmetrical  than  the  Gramme.  If  the 
two  electromotive  forces  were  not  equal,  the  armature 
would  be  like  two  batteries  having  unequal  electromotive 
forces  and  connected  in  multiple  arc,  in  which  case  the 
stronger  would  evidently  charge  the  weaker  on  open  cir- 
cuit. 

In  regard  to  the  magnetism  of  the  armature  and  the  dis- 
tribution of  the  iron,  the  following  are  among  the  most 
important  rules  to  be  guided  by.  As  the  space  between 
the  pole  piece  and  the  core  of  the  armature  is  composed  of 
the  non-magnetic  materials,  air  and  copper,  it  offers  the 
greatest  resistance  which  the  lines  of  force  have  to  pass 
through,  in  their  magnetic  circuit.  This  space  should 
therefore  be  as  thin  as  possible  measured  radially  to  the 
armature  or  in  the  direction  of  the  lines  of  force  ;  it  should 
also  have  as  large  a  surface  as  possible  measured  on  the 
cylindrical  surface  of  the  armature,  which  is  the  same  as 
the  active  surface  of  the  armature.  By  reducing  the 
thickness  of  this  space  by  one-half,  it  does  not  follow  as  is 
sometimes  supposed,  that  the  magnetism  will  be  four  times 
as  great,  as  the  well-known  rule  that  the  magnetic  inten- 
sity varies  inversely,  as  the  square  of  the  distance  does  not 
apply  to  the  distance  between  the  two  neighboring  sur- 
faces of  two  large  magnets.  Although  the  magnetism 
does  not  increase  inversely  as  the  square  of  this  distance, 
yet  it  is  increased  very  much  by  reducing  this  distance  be- 
tween the  armature  core  and  the  pole  pieces.  It  therefore 
follows  from  this  also,  that  the  number  of  windings  in  the 
armature  should  be  made  as  small  as  possible  to  reduce 
this  thickness  ;  also  that  the  thickness  of  the  copper  wire 


52  Principles  of  Dynamo- Electric  Machines. 

should  not  be  greater  than  is  necessary  to  carry  the  cur- 
rent without  heating  too  much.  The  diameter  of  the  arm- 
ature core  should  be  as  large  as  practicable,  and  its  length, 
or  the  length  of  the  pole  pieces,  should  be  as  great  as  prac- 
ticable, in  order  to  increase  the  area  of  this  non-magnetic 
space. 

Prof.  S.  P.  Thompson  has  made  some  experiments1  de- 
signed to  show  that  by  placing  iron  ridges  or  partitions  be- 
tween the  armature  coils  extending  from  the  iron  core  to 
near  the  pole  pieces,  the  electromotive  force  will  be  con- 
siderably greater.  As  the  number  of  windings  and  the 
speed  in  this  experiment  were  the  same  in  both  cases,  it 
follows  that  the  magnetism  must  have  been  thereby  in- 
creased. The  iron  projections  or  lugs  as  shown  in  his  illus- 
tration were  very  large  and  massive,  and  it  is  therefore  a 
question  whether  the  increased  effect  was  due  to  the  in- 
creased amount  of  iron  or  to  the  different  shape,  as  it  is 
possible  that  his  Gramme  armature  core  was  over  satur- 
ated. It  is  also  a  question  whether  the  electromotive 
force  would  not  be  increased  by  omitting  these  lugs  and 
filling  the  space  occupied  by  them  with  some  additional 
windings  on  the  armature.  This  could  readily  be  proved 
by  a  simple  experiment. 

The  lines  of  force  pass  through  the  cores  of  the  mag- 
nets, the  pole  pieces,  and  thence  through  the  armature  core 
as  shown  in  figure  8.  The  most  economical  cross  section 
of  these  parts  is,  as  explained  before,  that  at  which  the 
iron  is  just  saturated.  Supposing  that  the  field  magnet 
cores  have  been  so  proportioned,  the  cross  section  of  the 
armature  core  can  then  readily  be  calculated.  For  in- 
stance, if  there  are  four  magnets,  as  in  the  Weston  type  of 
frame,  the  iron  ring  of  the  Gramme  armature  should  evi- 
dently have  a  cross  section  equal  to  that  of  one  field  mag- 
net core;  for  as  many  lines  of  force  pass  through  one  half 
of  the  ring  as  through  one  field  magnet  core,  as  will  be 
shown  in  discussing  field  magnets.  This  supposes  the 

1.   Dynamo- Electric  Machinery  ;   pages  66-69. 


Armatures.  53 

quality  of  the  iron  to  be  the  same  in  both.  As  the  field 
magnets  are  generally  cast  iron,  while  the  armature  core  is 
wrought  iron,  its  cross  section  would  need  to  be  only 
about  two-thirds  as  great.  On  the  other  hand,  the  arma- 
ture core  is  not  solid  but  is  generally  laminated  or  made 
of  wire,  so  that  if  the  outside  cross  section  of  the  core 
when  completed  is  taken,  it  should  be  made  greater  than 
f  by  the  amount  of  the  non-magnetic  space  between  the 
laminae. 

The  so-called  Foucault  currents  in  an  armature  core  are 
nothing  more  than  currents  which  are  induced  according 
to  the  same  laws,  and  in  the  same  direction  as  the  useful 
currents  in  the  copper  wire,  only  that  they  are  induced  in 
the  iron  where  they  cannot  be  used;  for  it  is  evident  that 
induction  will  take  place  in  any  moving  metallic  parts  of 
the  armature  which  cut  lines  of  force,  whether  these  parts 
be  copper  conductors  or  masses  of  iron  forming  the  mag- 
netic core.  It  will  be  seen  from  this  that  the  Foucault 
currents  tend  to  flow  in  the  core  under  the  windings,  in 
the  same  direction  as  the  useful  currents,  that  is,  parallel 
to  the  axis,  and  in  two  opposite  directions  on  the  two 
sides.  They  are  most  intense  in  the  outer  portions  of  the 
core,  as  the  speed  and  number  of  lines  of  force  cut  are 
greatest  there.  By  omitting  the  core  and  winding  a 
Gramme  armature  entirely  of  coils  of  iron  wire,  connect- 
ing them  to  the  commutator  as  usual,  the  Foucault  cur- 
rents then  become  the  useful  currents  and  will  be  the 
only  currents  induced.  In  this  case  the  iron  wire  coils 
act  at  the  same  time  as  a  magnetic  core,  and  as  conduc- 
tors of  the  current.  As  the  resistance  of  iron  wire  is  so 
high,  this  method  does  not  appear  to  have  been  a  success 
in  practice.  It  is  given  here  merely  as  an  illustration  of 
the  nature  of  Foucault  currents. 

As  with  the  useful  currents,  the  Foucault  currents  are 
the  result  of  an  induced  electromotive  force  equalizing  it- 
self through  a  certain  resistance.  To  prevent  these  cur- 
rents from  being  generated  it  is  necessary  either  to  prevent 


54  Principles  of  Dynamo-Electric  Machines. 

the  induction  of  the  electromotive  force,  or  to  pre- 
vent the  electromotive  force  from  equalizing  itself  by 
making  the  resistance  infinitely  great.  The  former  is  im- 
practicable, for  the  only  way  to  prevent  the  induction  of 
electromotive  force  in  the  core  is  either  to  keep  the  core 
fixed  in  position  so  as  not  to  cut  lines  of  force,  or  else  to 
omit  the  core  altogether  or  make  it  of  wood  or  some 
other  non-conductor.  The  only  practical  way,  therefore, 
to  avoid  the  generation  of  the  currents  is  to  make  the  re- 
sistance very  great  by  laminating  or  otherwise  dividing  the 
iron  core  into  a  large  number  of  small  insulated  parts. 
As  the  direction  of  these  currents  is  the  same  as  that  in 
the  copper  wires,  this  lamination  must  be  perpendicular  to 
the  copper  wires,  that  is  perpendicular  to  the  axis.  An 
armature  core  should  therefore  be  made  of  discs,  rings,  or 
wires  in  planes  perpendicular  to  the  axis,  and  insulated 
from  each  other  so  as  to  prevent  currents  from  flowing 
parallel  to  the  axis  or  the  copper  conductors.  The  induc- 
tion of  small  electromotive  forces  will  nevertheless  take 
place  in  each  disc,  but  they  will  not  be  connected  in  series 
and  cannot  produce  currents;  they  will  therefore  not  con- 
sume energy,  nor  heat  the  core,  as  energy  is  electromotive 
force  multiplied  by  current.  The  discs  should  be  as  thin 
as  possible,  for  if  they  are  too  thick  the  unequal  induced 
electromotive  forces  in  different  parts,  which  are  moving 
at  different  speeds,  will  cause  currents  to  circulate  in  each 
disc. 

The  cores  should  be  made  of  the  softest  wrought  iron, 
to  diminish  the  amount  of  magnetic  lag  mentioned  before. 
In  well  proportioned  and  well  designed  machines  they 
should  never  be  made  of  cast  iron,  even  if  it  is  cast  mas- 
sive, and  then  cut  so  as  to  be  a  substitute  for  laminae. 
Cast  iron  has  a  lower  saturation  point  than  wrought  iron, 
therefore  cores  of  cast  iron  require  a  larger  bulk  of  iron, 
than  when  made  of  wrought  iron.  This,  at  the  same  time, 
will  increase  the  length  and,  therefore,  the  resistance  of  the 
copper  wire,  thereby  decreasing  the  efficiency  of  the 


Armatures.  55 

machines.  A  core  of  a  Gramme  or  cylinder  armature  should 
under  no  circumstances  be  solid  iron. 

If  the  laminae  are  to  be  made  of  rolled  wrought  iron, 
it  will  generally  be  found  that  the  iron  sheets  are  coated 
with  a  thin  layer  of  blue  oxide.  This  is  very  hard  and 
acts  to  retard  the  magnetization  and  demagnetization. 
This  should  be  removed,  which  can  readily  be  done  by 
immersing  the  discs  for  a  short  time  in  dilute  sulphuric 
or  hydrochloric  acid  (about  1  to  10),  after  which  they 
should  be  washed  very  thoroughly  in  water,  and  prefer- 
ably immersed  in  a  bath  containing  soda  or  potash,  other- 
wise the  acid  will  rust  them  badly  after  the  armature  is  made. 

In  order  to  see  whether  this  blue  oxide  retarded  the  de- 
magnetization, the  writer  made  the  following  experiment: 
Several  discs  were  taken  from  the  same  lot;  some  of  them 
were  cleaned  as  described,  while  the  others  were  left  with 
the  coating  of  blue  oxide  on  them.  They  were  fastened 
together  and  magnetized  very  strongly  in  one  direction, 
all  being  magnetized  to  the  same  degree.  After  this  they 
were  examined  for  magnetism,  and  it  was  found  that  the 
clean  ones  had  very  little  residual  magnetism,  while  the 
others  showed  strong  magnetic  polarity. 


CHAPTER   V. 

Armatures. — ( Continued?) 

IN  chapter  iv.  it  was  stated  that  the  wire  in  the  inside 
of  the  ring  was  a  dead  resistance,  as  there  is  no  induction 
in  it.  Numerous  devices  have  been  used  to  render  it 
active,  among  which  are  the  following  : 

The  pole  pieces  have  been  made  to  embrace  almost  the 
entire  surface  of  the  ring,  by  shaping  the  ends  so  as  to 
project  into  the  inside  of  the  ring,  as  shown  in  figure  9. 


9 


This  will  increase  the  intensity  of  the  lines  of  force  some- 
what, because  it  decreases  the  magnetic  resistance  of  the 
circuit  of  the  lines  of  force  by  increasing  the  active  area 
of  the  two  magnetic  surfaces  of  the  core  and  the  pole 
pieces,  between  which  the  lines  of  force  have  to  pass. 
For  instance,  if  this  surface  is  doubled  by  these 

(56) 


Armatures.  57 

pole-piece  extensions  it  is  equivalent  to  reducing  the 
thickness  of  this  air  space  about  one-half,  and  thereby  re- 
duces its  resistance  to  one-half.  If  the  surface  of  the  pole 
pieces  is  not  over-saturated  without  these  extensions,  then 
by  adding  them,  thereby  doubling  its  area,  for  instance, 
the  magnetism  of  the  field  magnets  may  also  be  doubled, 
provided  the  rest  of  the  iron  of  the  magnets  be  increased 
so  as  not  to  be  over-saturated  This  will  double  the  num- 
ber of  lines  of  force,  and,  therefore,  also  the  electromotive 
force  generated.  This  method  of  making  the  wire  active 
is,  however,  not  to  be  recommended,  for  by  adding  these 
extensions  the  construction  is  complicated  very  much.  It 
will  be  seen  from  the  figure  that  unless  the  armature  is 
very  large  in  diameter,  many  of  the  lines  of  force  would 
pass  directly  from  the  points  a  a  of  one  of  the  pole  pieces 
through  the  iron  shaft  to  the  opposite  pole  piece,  which 
would  evidently  be  leakage,  and  represent  so  much  wasted 
magnetism.  This  leakage  might  even  be  greater  than  the 
small  advantage  gained  by  the  extensions. 

Another  device  for  rendering  the  dead  wire  active,  is  to 
place  in  the  inside  of  the  ring  an  additional  fixed  electro- 
magnet, whose  poles  are  situated  opposite  to  the  other  pole 
pieces,  and  have  the  same  polarity  as  the  corresponding 
pole  pieces  on  the  outside,  thus  making  the  wire  on  the  in- 
side cut  lines  of  force  in  the  proper  direction.  If  this 
magnet  is  not  very  powerful  it  will  short  circuit  magneti- 
cally the  outside  pole  pieces,  and  in  that  case  do  more 
harm  than  good.  This  device,  as  well  as  the  one  above- 
mentioned,  is  impracticable,  as  it  complicates  the  construc- 
tion very  greatly. 

The  best  method,  and  one  which  has  proved  to  be  very 
successful  in  practice,  is  to  make  the  ring  flat,  like  a  disc 
with  a  large  hole  in  it,  the  pole  pieces  being  then  at  the 
two  flat  sides.  Electrically,  this  is  equivalent  to  putting 
an  extra  magnet  in  the  inside  of  the  ordinary  form  of 
Gramme  ring,  as  it  renders  both  of  the  long  sides  of 
the  rectangular  armature  coils  active,  and  it  has  moreover 


58  Principles  of  Dynamo-Electric  Machines. 

the  advantage  that  it  does  not  complicate  the  con- 
struction. These  so-called  "  flat-ring "  machines  are 
used  very  largely  in  Germany.  The  Brush  armature  is 
of  this  type,  but  has  its  coils  connected  differently. 
The  most  economic  form  of  cross  section  of  such  a  flat- 
ring  machine  would  appear  to  be  a  rectangle  in  which 
the  long  side  is  two  or  three  times  the  shorter  ;  by  making 
it  much  more  than  this  the  diameter  of  the  armature  is 
increased  too  much,  or  else  the  speed  of  those  parts  of  the 
coils  which  are  nearest  the  axis  is  too  small. 

In  regard  to  the  shape  of  the  cross  section  of  the  ordi- 
nary Gramme  armature  core,  no  general  rules  can  be  given, 
as  it  will  depend  greatly  on  the  general  construction  of  the 
machine.  The  size  of  the  cross  section  having  been  determ- 
ined as  described  in  considering  the  magnetic  effects,  it  is 
well  to  make  the  length  parallel  to  the  axis  as  great 
as  practicable  .and  the  thickness  correspondingly  small,  for 
by  doing  so  the  required  length  of  the  wire  on  the  arma- 
ture will  make  less  windings  in  a  coil,  thereby  dimin- 
ishing the  self  induction,  the  sparking  and  the  thickness  of 
the  non-magnetic  space  between  the  core  and  the  pole 
pieces,  besides  increasing  its  area.  For  the  same  reason,  if 
it  is  required  to  increase  the  cross  section  of  the  armature, 
it  is  in  general  preferable  to  increase  the  length  parallel  to 
the  shaft  rather  than  to  increase  the  thickness.  pFor  a  given 
thickness,  the  magnetism  increasing  in  proportion  to  the 
length  of  the  armature  core,  it  is  evident  that  the  electromo- 
tive fojrce  will  be  approximately  proportional  to  the  length 
of  the  core,  other  conditions  being  the  same?}  This  rule  is 
merely  a  different  statement  of  the  rule  given  before,  that 
the  electromotive  force  is  proportional  to  the  magnetism 
passed  through  by  the  wire,  it  being  understood  that  the 
iron  of  the  armature  and  magnets  is  also  increased  in 'the 
same  proportion,  in  order  to  be  saturated  to  the  same  de- 
gree. 

For  a  given  cross  section  of  the  core  the  distance  around 
the  outside,  that  is  the  length  of  one  winding,  will  be  least, 


Armatures.  59 

if  the  cross  section  is  circular.  It  might  therefore  appear 
that  this  is  the  best  shape  of  the  cross  section.  If  the 
magnets  and  pole  pieces  were  of  such  a  shape  as  to  sur- 
round this  circle  as  completely  as  possible,  it  would  no 
doubt  be  an  economical  form  ;  but  at  the  same  time  it 
greatly  increases  the  difficulties  of  construction,  and  as  it 
is  difficult  to  make  the  windings  on  the  armature  as 
smooth  and  regular,  it  necessitates  an  increased  distance 
between  the  core  and  the  pole  pieces,  which  is  a  very  bad 
feature.  In  general,  ^he  cross  section  will  depend  on  the 
general  style  of  the  machine,  and  therefore  the  best  shape 
should  be  found  ty  choosing  from  calculations  made  for 
several  assumed  shapes. 

The  core  of  an  armature  should  not  be  hollow,  for  it  is 
evident  that  if  the  hollow  core  is  not  over-saturated,  the 
length  of  the  wire  around  it,  and,  therefore,  its  resistance, 
could  be  made  less  by  making  the  cross  section  solid  but  of 
the  same  area  of  cross  section  of  iron  as  before.  On  the 
other  hand,  if  it  is  over-saturated  when  hollow,  the  mag- 
netism will  be  increased  by  filling  the  hollow  space  within 
it  with  iron,  thus  making  it  solid. 

The  diameter  of  the  armature  should  evidently  be  as 
great  as  practicable,  for  by  increasing  it  other  parts  may 
be  decreased.  For  a  given  electromotive  force,  all  other 
conditions  being  the  same,  an  increase  in  the  diameter  will 
affect  the  other  proportions  as  follows  :  the  speed  may  be 
decreased  in  the  same  proportion  ;  the  number  of  windings 
or  the  length  of  wire  (in  a  Gramme  armature),  and,  there- 
fore, also  the  distance  between  the  armature  core  and  the 
pole  pieces,  may  be  decreased  in  about  the  same  propor- 
tion ;  or  the  latter  may  remain  the  same  and  the  cross 
section  of  the  wire  increased,  thus  increasing  the  allowable 
current  of  the  machine  in  about  the  same  proportion  ; 
either  the  intensity  or  the  size  (volume)  of  the  mag- 
nets may  be  decreased  in  about  the  same  proportion  ; 
besides  these,  the  distance  between  the  projecting  ends 
of  opposite  pole  pieces  may  be  increased  in  the  same 


60  Principles  of  Dynamo-Electric  Machines. 

proportion,  thus  decreasing  the  leakage  of  magnetism;  the 
length  of  the  armature  core  may  be  decreased  in  the  same 
proportion  ;  the  weight  of  the  armature  core  will  be 
increased  in  the  same  proportion,  in  the  case  of  a  Gramme 
ring-  with  a  certain  cross  section  of  core,  while  in  the  case 
of  a,  cylinder  armature  the  weight  will  be  increased  as  the 
square  of  the  diameter. 

From  the  last  two  statements,  and  from  the  fact  that 
the  whole  machine  is  sometimes  increased  in  size  with 
a  larger  diameter,  it  is  seen  that  a  limit  is  soon  reached  to 
which  the  diameter  may  be  economically  increased.  In 
this  case,  as  well  as  in  the  case  of  several  other  proportions, 
it  is  best  to  assume  different  values  for  the  diameter,  and 
calculate  the  other  parts  affected  by  it  for  each  case,  from 
which  the  best  proportions  can  then  be  readily  chosen. 

The  general  rule  in  regard  to  speed  or  number  of  revo- 
lutions, is  to  make  it  as  great  as  practicable.  Its  limits 
are,  in  practice,  purely  mechanical,  as  there  is  no  reason, 
electrically,  why  the  speed  should  not  be  very  great, 
except  that  the  electromotive  force  of  the  Foucault  cur- 
rents and  the  resistance  to  changes  of  magnetism  in 
the  armature  core  are  thereby  greater,  thus  requiring 
finer  laminations  of  the  iron  and  softer  iron  in  the  core  ;  but 
neither  of  these  two  will  be  as  important  in  limiting  the 
speed  as  the  mechanical  considerations.  For  a  given  elec- 
tromotive force,  all  other  conditions  being  the  same,  an 
increase  in  the  speed  will  affect  the  other  proportions  as 
follows:  the  diameter  may  be  decreased  in  the  same  pro- 
portion ;  the  number  of  windings  or  length  of  wire  on  the 
armature,  and,  therefore,  also  the  distance  between  the 
armature  core  and  the  pole  pieces,  may  be  decreased  in 
about  the  same  proportion  ;  or  the  latter  may  remain  the 
same  and  the  cross  section  of  the  wire  increased,  thereby 
increasing  the  allowable  current  in  about  the  same  propor- 
tion ;  either  the  intensity  or  the  size  (volume)  of  the  mag- 
nets may  be  decreased  in  about  the  same  proportion  ;  and 
in  conjunction  with  this  the  breadth  of  the  pole  pieces 


Armatures.  61 

measured  on  the  circumference  of  the  armature  may  be 
decreased  if  they  are  not  over-saturated,  thus  increasing 
the  distance  between  the  projecting  ends  of  the  two 
opposite  pole  pieces  and  thereby  decreasing  the  leakage  of 
the  magnetism  between  these  parts  ;  if  the  amount  of 
magnetism  is  decreased,  either  the  length,  the  thickness  or 
the  cross  section  of  the  armature  core,  or  what  is  the  same 
thing,  the  weight  of  the  armature  core  may  be  decreased 
in  about  the  same  proportion. 

The  mechanical  considerations  for  high  speed  are  as  fol- 
lows :  The  armature  should  be  as  light  as  possible.  Its 
diameter  should  not  be  too  great.  The  shaft  should  be 
large  in  diameter  to  resist  all  tendency  to  bending.  The 
length  of  the  shaft  between  the  bearings  should  be  as  short 
as  possible  to  prevent  bending,  and,  therefore,  vibrating  and 
causing  a  trembling  of  the  machine.  The  bearing  at  the 
pulley  end  should  therefore  be  between  the  pulley  and  the 
armature,  not  outside  of  the  pulley.  The  armature  and 
commutator  between  the  two  bearings  should  be  as  short 
as  possible,  and  there  should  be  no  lost  space  between  them 
or  the  bearings.  The  bearings  should  be  long,  at  least  three 
to  five  times  the  diameter  of  the  shaft  ;  and  they  should  be 
made  of  some  anti-friction  metal ;  in  no  case  should  bear- 
ings be  of  iron  when  the  shaft  is  of  iron  or  steel,  on  ac- 
count of  the  tendency  of  the  magnetic  attraction  to  in- 
crease the  friction  by  pressure.  The  bearings  should  be 
as  free  from  magnetism  as  possible,  to  avoid  the  genera- 
tion of  Foucault  currents  in  the  shaft,  which  are  necessa- 
rily short-circuited  through  the  centre  of  the  shaft,  and 
therefore  may  be  very  intense,  and  then  heat  the  bearings. 
The  bearings  should  be  bored  as  true  as  possible  and  pre- 
ferably bored  at  the  same  time  with  the  pole  pieces.  They 
should  preferably  be  fastened  directly  to  the  pole  pieces 
and  not  bolted  on  a  base  plate  on  which  the  machine  is 
fastened,  as  they  are  then  liable  to  be  out  of  line.  The 
supports  of  the  bearings  should  be  as  rigid  as  possible  to 
prevent  vibrating  or  trembling.  It  is  of  special  importance 


Principles  of  Dynamo-Electric  Machines. 


in  high  speed  that  the  armature  should  be  rigidly 
connected  to  its  shaft,  and  that  it  should  be  well  bound  to 
prevent  bursting,  or  to  prevent  the  wires  from  bending 
outward  by  centrifugal  force,  thereby  coming  into  con- 
tact with  the  pole  pieces  ;  the  tie  bands  around  the  arma- 
ture wire  should  therefore  be  very  tight  and  strong,  and 
not  too  far  apart.  The  armature  should  be  as  smooth  as 
possible  on  the  outside  to  prevent  churning  the  air.  The 
commutator  should  have  as  small  a  diameter  as  prac- 
ticable to  prevent  too  much  motion  between  it  and  the 


.  XO 


brushes.  It  is  preferable  in  cases  of  high  speed  to  allow 
the  shaft  to  have  a  slight  lateral  play  of  -J-  or  \  inch,  as  it 
then  distributes  the  oil  better  and  is  not  so  apt  to  cut  the 
bearing.  In  this  case  the  machine  should  be  shifted  on 
its  foundation  while  running  with  its  full  load,  until  the 
shaft  has  a  free  lateral  motion,  as  the  position  of  the  ma- 
chine to  allow  this  free  lateral  motion  may  be  diiferent 
when  running  with  a  full  load  than  when  running  without 


Armatures.  63 

a  load,  on  account  of  the  magnetic  attraction  of  the  arma- 
ture and  its  field  magnets.  The  distance  between  the 
armature  core  and  the  pole  pieces  should  be  precisely  the 
same,  on  both  sides,  otherwise  the  armature  will  be  at- 
tracted with  very  great  force  to  one  pole  piece  when  the 
field  is  strong,  thus  bending  the  shaft  and  heating  the 
bearings.  As  the  oil  has  a  greater  tendency  to  get  on  to 
the  armature  and  commutator  when  the  speed  is  great,  it 
is  preferable  in  that  case  to  have  an  oil  catcher  or  screen 
of  some  sort  between  the  bearing  and  the  armature  and 
commutator.  The  simplest  form  is  to  make  a  groove  in  the 
inside  of  the  bearing  with  a  dripping  tube  at  the  bottom, 
as  shown  at  a  in  figure  10.  Another  simple  and  effective 
device  is  to  fasten  a  thin  disc  to  the  shaft  near  the  inside 
end  of  the  bearing,  as  shown  at  d,  figure  10.  The  oil  must 
pass  around  this  disc  to  get  to  the  armature,  and  as  the 
disc  revolves  with  the  shaft  the  oil  is  thrown  off  at  the 
edge  by  centrifugal  force,  and  will  therefore  not  get  on  to 
the  other  side.  It  may  be  caught  and  collected  by  a 
grooved  ring  and  dripping  tube,  as  shown. 

Besides  these  considerations  in  connection  with  high 
speed,  it  is  especially  necessary  to  have  the  armature  per- 
fectly balanced  mechanically,  for  if  it  is  not  balanced  it 
will  be  liable  to  vibrate,  which  will  be  augmented  by  the 
consequent  unbalanced  magnetic  pull  of  a  strong  field  on 
the  armature,  due  to  the  core  vibrating,  so  as  to  come 
nearer  to  one  pole  piece  and  farther  from  the  other.  This 
vibration  may  become  great  enough  to  abrade  the  surface 
of  the  armature  on  the  pole  pieces,  which  is  apt  to  cut  the 
tie  bands,  resulting  in  the  short  circuiting  and  the  total 
destruction  of  the  armature.  To  balance  the  armature 
statically,  place  it,  when  quite  completed,  on  two  horizon- 
tal "  knife  edges"  so  that  it  rests  near  the  two  ends  of  the 
shaft.  If  on  rolling  it  slightly  it  comes  to  rest  in  any  po- 
sition, it  is  balanced,  if  not,  weights  must  be  added  se- 
curely, or  better,  metal  must  be  removed  if  possible,  so 
that  it  will  come  to  rest  in  any  position.  It  is  often 


64 


Principles  of  Dynamo- Electric  Machines. 


thought  that  this  method  of  balancing  is  sufficient,  but 
this  is  not  the  case,  for  an  armature  may  be  balanced  per- 
fectly on  knife  edges  and  yet  it  will  vibrate  when  running 
at  high  speed.  This  will  be  seen  from  figure  11.  Suppose 
there  is  an  excess  of  weight  at  w  and  an  equal  amount  at  a 
diametrically  opposite  point  w'  but  at  the  other  end.  It 
will  be  balanced  perfectly  on  the  knife  edges,  but  when 
revolving  there  will  evidently  be  a  tendency  to  distort  the 


shaft  and  armature  as  shown  in  dotted  lines,  which  will 
tend  to  cause  vibrations  at  the  bearings.  It  is  similar  to 
the  case  of  a  bicycle  wheel  which  may  be  balanced  stat- 
ically or  when  revolving  slowly,  but  when  revolving 
rapidly  it  will  tend  to  vibrate,  which  is  due  to  the  weight 
of  the  cranks  and  pedals  at  two  diametrically  opposite 
parts  at  the  two  ends.  In  the  case  of  an  armature,  it  may 
be  detected  in  several  different  ways,  one  of  which  is 
to  revolve  it  rapidly  while  resting  on  two  bearings,  which 
are  suspended  and  are  free  to  move  laterally. 

An  increase  in  speed  represents  a  saving  of  material  and 
a  decrease  of  resistance  in  the  armature  and  magnets  ;  it  is, 
therefore,  a  direct  gain,  both  in  cost  of  material  and  to  a  ( 
certain  extent  in  efficiency,  and  consequently  it  is  of  equally 
great  importance  to  take  as  much  care  with  the  mechanical 
details  of  construction  as  with  the  purely  electrical  propor- 
tions. It  is  much  easier  to  build  a  low  speed  dynamo,  as 
defects  in  its  construction  may  exist  which  would  be  fatal 


Armatures.  65 

to  the  successful  running  of  a  high  speed  dynamo.  The 
aim  of  the  technical  engineer  is  to  lind  the  most  economical 
proportions,  while  the  aim  of  the  manufacturer  is  to  find 
the  cheapest  form.  As  the  proportions  and  the  cost  of 
material  both  decrease  with  an  increased  speed,  the  im- 
portance of  careful  attention  to  necessary  details  of  con- 
struction limiting  the  speed,  becomes  evident.  Many  me- 
chanical machines  are  in  continued  daily  use  which  are 
run  successfully  at  a  much  higher  speed  than  dynamos. 
The  success  of  a  certain  well-known  machine  which  is  elec- 
trically deficient,  is  mainly  due  to  the  exceptionally  high 
speed  at  which  it  can  be  safely  run. 


CHAPTER    V. 

Armatures. — ( Continued.) 

THE  electromotive  force  induced  in  the  armature  de- 
pends directly  on  the  velocity  with  which  the  active  parts 
of  the  wire  move  through  the  field.  This  is  commonly 
known  as  the  "  conductor  velocity,"  though  it  ought  prop- 
erly to  be  called  the  "  inductor  velocity,"  as  the  wires 
moving  through  the  magnetic  field  are  the  "inductors,"  or 
those  in  which  the  induction  takes  place;  while  the  wires 
at  the  ends,  and  in  the  inside  of  a  Gramme  ring,  are  the 
conductors,  as  their  only  function  is  to  conduct  the  current 
which  is  generated  in  the  inductors.  This  inductor  veloci- 
ty will  be  greater  as  the  distance  of  the  moving  wire  from 
the  shaft  is  increased,  and  as  the  number  of  revolutions  is 
increased;  in  other  words,  the  inductor  velocity  depends 
both  on  the  diameter  (or  radius)  of  the  armature,  and  on 
the  number  of  revolutions;  it  is  not  a  function  of  either 
alone,  but  depends  on  their  product. 

This  inductor  velocity  should  evidently  be  as  great  as  it 
is  practical  to  make  it,  for  increasing  it  represents  a  direct 
gain  in  other  parts.  It  is  usually  measured  in  feet  per  sec- 
ond, and  may  be  calculated  by  multiplying  the  circumfer- 
ence of  the  armature,  in  feet,  by  the  number  of  revolutions 
per  second.  As  the  diameter  of  an  armature  is  usually 
given  in  inches,  and  the  speed  in  revolutions  per  minute, 
the  calculation  may  be  simplified  by  multiplying  the  diam- 
eter in  inches  by  the  speed  in  revolutions  per  minute,  and  by 
.0044  approximately  (accurately,  .004363),  which  gives 'the 
result  directly  in  feet  per  second.  If  greater  accuracy  is 
desired  than  that  obtained  by  using  the  external  circum- 
ference of  the  armature,  allowance  should  be  made  for  the 
thickness  of  the  layers  of  wire,  as  the  inside  wires  have  a 

(66) 


Armatures.  67 

somewhat  less  velocity;  therefore,  instead  of  taking  the 
outside  circumference  of  the  armature,  it  is  better  to  take 
the  mean  of  the  circumferences  at  the  outside  and  at  the 
inside  layer. 

As  this  inductor  velocity  is  dependent  on  the  product  of 
the  diameter  and  the  speed,  it  may  be  increased  by  in- 
creasing either  of  these,  but  from  the  mechanical  consid- 
erations it  is  preferable  to  increase  the  diameter  rather  than 
the  speed,  because  in  doubling  the  inductor  velocity,  for 
instance,  it  is  easier  to  double  the  diameter  and  run  at 
the  same  number  of  revolutions,  than  it  is  to  keep  the  same 
diameter  and  run  at  double  the  speed.  There  is  also  an- 
other reason  why  this  is  preferable,  for  by  doubling  the 
diameter,  and,  therefore,  the  circumference  or  surface  of 
the  armature,  the  same  number  of  turns  of  wire  will  make 
only  half  as  many  layers,  thus  decreasing  the  distance  be- 
tween the  pole  pieces  and  the  core,  and  very  greatly  de- 
creasing the  proportion  of  this  distance  to  the  diameter  of 
the  armature.  An  armature  of  small  diameter  is,  therefore, 
running  at  a  disadvantage,  for  with  the  same  length  of 
armature,  same  speed,  current  and  number  of  turns  on  the 
armature,  the  electromotive  force,  and  therefore  the  capac- 
ity of  the  machine,  may  be  doubled,  and  at  the  same  time 
the  number  of  layers  halved,  by  doubling  the  diameter  of 
the  armature,  the  only  other  change  being  a  larger 
field  to  embrace  double  the  area  of  armature  surface,  and 
a  somewhat  greater  amount  of  dead  resistance. 

The  best  guide  in  selecting  the  inductor  velocity  to  be 
used  in  dynamos,  is  to  compare  the  results  of  practice 
in  well-built  machines.  In  a  number  of  the  best  Edison 
machines1  it  varied  from  about  46  to  54  feet  per  second, 
for  cylinder  armatures  varying  from  10-j-  to  7  inches  in 
diameter,  the  speed  varying  from  1,100  to  1,600  revolu- 
tions per  minute.  With  the  latter  high  speed  the  inductor 
velocity  was  only  46.5  feet  per  second  for  a  7-inch 

1.  See  table  in  Appendix  I. 


68  Principles  of  Dynamo-Electric  Machines. 

armature,  showing,  as  stated  above,  that  small  armatures  do 
not  run  as  advantageously  as  larger  ones.  In  several  of 
the  best  Weston  incandescent  light  machines,  with  8  to 
9-inch  armatures  and  speeds  from  1,050  to  1,250  revolu- 
tions per  minute,  the  mean  inductor  velocity  varied  from 
37  to  43  feet  per  second;  while  for  some  Weston  arc-light 
machines  it  was  as  high  as  61  feet  per  second.  In  a  small 
Weston  incandescent  light  machine  with  a  4^-inch  arma- 
ture, at  1,380  revolutions  per  minute,  the  mean  inductor 
velocity  was  only  26  feet  per  second.  A  contrast  to  this 
small  armature  is  the  large  60  arc  light  Brush  machine, 
which  makes  only  825  revolutions  per  minute,  but  has  the 
very  high  mean  inductor  velocity  of  72  feet  per  second, 
the  mean  diameter  being  about  20  inches,  which  shows  the 
great  advantage  of  large  diameters  of  the  armatures.  An- 
other flat-ring  machine  (Schuckert)  had  65  feet  per  second. 
Hospitaller  states  that  66  to  82  feet  per  second  for  the 
middle  part  of  the  armature,  is  rarely  exceeded;  this,  no 
doubt,  has  reference  to  Gramme  armatures,  and  gives  for 
the  inductor  velocity  about  83  to  100  feet  per  second.  A 
Siemens  cylinder  armature  machine  had  as  high  as  107 
feet  per  second,  but  this,  as  well  as  the  previous  one,  no 
doubt  exceeds  good  engineering  practice.  As  a  rule, 
Gramme  armatures  are  lighter  and  larger  in  diameter,  and 
can  therefore  be  run  at  a  higher  inductor  velocity  than 
cylinder  armatures,  which  is  an  important  advantage  over 
the  latter. 

One  of  the  principal  proportions  in  armatures  is  the  re- 
lation between  the  length  of  the  armature  wire  and  the 
electromotive  force  which  it  is  to  generate.  Although 
this  is  of  such  great  importance,  there  is,  strange  to  say, 
little  or  no  practical  information  regarding  it  given  in  Ithe 
numerous  text-books.  Elaborate  theories  and  complicated 
formulae  have  been  published,  but  it  is  doubtful  whether 
they  are  of  much  value  in  practice.  The  most  reliable  pro- 
portions are,  no  doubt,  those  which  may  be  deduced  from 
some  of  the  best  existing  machines.  The  writer  has, 


Armatures.  69 

therefore,  calculated  some  constants  from  the  proportions 
of  some  Weston  and  Edison  machines,  which,  though  taken 
from  entirely  different  machines — both  arc  and  incandes- 
cent— agree  so  well  that  they  can  safely  be  taken  as  a  re- 
liable guide  in  determining  what  length  of  wire  is  required 
in  the  armature,  to  give  a  certain  required  number  of 
volts.  These  machines  were  all  of  the  cylinder  armature 
type,  but  it  is  presumed  that,  if  properly  applied,  the  con- 
stants may  also  be  used  for  Gramme  ring  armatures. 

As  stated  in  a  previous  chapter,  the  electromotive  force 
is  induced  when  a  wire  cuts  lines  of  force,  and  therefore 
only  that  part  of  the  wire  is  generating  electromotive 
force  which  lies  directly  between  the  pole  pieces  and  the 
armature  core.  This  will  be  termed  "  active  wire,"  in  dis- 
tinction to  the  dead  wire  at  the  ends  of  the  armature,  and 
that  which  is  not  embraced  by  the  pole  pieces.  A  few 
lines  of  force  pass  around  from  the  outside  of  the  pole 
pieces  to  the  ends  of  the  armature,  thus  rendering  part  of 
the  otherwise  dead  wire  active  ;  some  also  pass  obliquely 
from  the  thin  ends  of  the  pole  piece  projections  to  the 
armature  core,  thus  rendering  active  some  of  the  wires 
which  are  not  directly  between  the  iron  of  the  pole  pieces 
and  the  armature  core.  But  as  the  intensity  of  the  field 
is,  or  should  be,  so  very  much  greater  in  the  space  lying 
directly  between  the  pole  pieces  and  the  armature  core, 
the  small  amount  of  induction  in  "the  other  parts  of  the 
wire  may  be  neglected.  With  this  assumption,  it  was 
found  that  in  several  Weston  incandescent  light  machines 
the  total  electromotive  force  generated  was  at  the  rate  of 
from  1  to  1.3  volts  for  every  foot  of  active  wire,  while  from 
some  rough  measurements  of  several  arc  light  machines  of 
the  same  type,  it  was  from  2.2  to  3.2  volts  per  foot,  with  a 
higher  inductor  velocity  and  a  more  intense  field.  In  sev- 
eral Edison  machines,  of  different  sizes,  it  was  from  1.5  to 
1.8  volts  per  foot.  It  is  assumed,  in  deducing  these  con- 
stants, that  the  field  is  uniform  in  all  parts;  this  is  probably 
not  the  case,  but  it  will  not  materially  affect  the  values 


70  Principles  of  Dynamo- Electric  Machines. 

of  the  constants,  as  they  may  be  taken  to  represent  the 
average  value  for  all  the  wires.  Moreover,  in  calculating 
armatures,  these  constants  are  to  be  applied  to  conditions 
similar  to  those  under  which  they  were  deduced,  and  the 
error  will,  therefore,  be  eliminated. 

The  electromotive  forces  induced  per  foot,  as  just 
given,  depend  on  the  speed  as  well  as  on  the  field.  In  or- 
der, therefore,  to  properly  compare  the  results,  and  to  re- 
duce them  to  a  form  in  which  they  may  be  used  for  differ- 
ent speeds,  it  is  necessary  to  eliminate  the  speed,  or,  in 
other  words,  to  divide  these  constants  by  the  inductor  ve- 
locity in  each  case,  thus  reducing  them  to  the  number  of 
volts  which  would  be  generated  if  the  active  wire  on  the 
armature  had  a  uniform  velocity  of  one  foot  per  second.  In 
the  Weston  incandescent  light  machines  this  reduced  con- 
stant was  .025  to  .030,  which  means  that  in  every  foot  of 
active  wire  on  the  armature,  .025  to  .030  volts  would  be 
generated  if  the  wire  had  a  velocity  of  one  foot  per  second. 
As  the  electromotive  force  is  directly  proportional  to  the 
speed,  this  constant  must  be  multiplied  by  the  velocity  of 
the  moving  wire  of  any  particular  armature,  in  feet  per  sec- 
ond, to  give  the  number  of  volts  per  foot  which  will  be 
generated  in  that  armature  when  running  at  the  desired 
speed.  In  several  Weston  arc  light  machines,  a  rough 
measurement  gave  from  .044  to  .052  volts  per  foot  for  an 
inductor  velocity  of  one  foot  per  second,  showing  that  the 
field  was  still  more  intense  than  in  the  incandescent  light 
machines.  In  several  Edison  incandescent  light  machines 
of  different  sizes,  it  was  .033  to  .037,  showing  that  the 
field  was  more  intense  than  in  the  Weston  incandescent 
light  machines,  but  not  as  intense  as  in  the  arc  light 
machines. 

As  these  constants  agree  tolerably  well  with  each  oth,er, 
they  may  be  safely  used  in  designing  armatures  for  ma- 
chines where  the  general  construction  is  similar  to  these 
machines.  Probably  they  could  also  be  used  for  Gramme 
armatures,  provided  that  only  the  active  wire  is  considered, 


Armatures.  71 

and  that  the  magnetic  field  can  be  made  as  intense  as  in 
these  Weston  and  Edison  machines.  The  core  of  the 
Gramme  armatures  will,  therefore,  in  most  cases,  have  to 
be  made  larger  than  is  customary,  in  order  not  to  weaken 
the  field  by  interposing  too  much  magnetic  resistance  in 
the  armature.  How  to  apply  these  constants  in  designing 
armatures  for  generating  a  required  electromotive  force, 
will  be  more  fully  explained  and  illustrated  in  a  subse- 
quent chapter  on  the  calculations  of  armatures. 

These  constants  evidently  depend  directly  on  the  in- 
tensity of  the  field,  and  vary  with  it,  for  if  the  intensity  of 
the  field  were  doubled,  the  values  of  these  constants — that 
is,  the  number  of  volts  per  foot — would  also  be  doubled. 
The  magnetic  fields  of  dynamos  will  be  considered  later; 
it  will  suffice  to  state  here  that  the  fields  of  the  .Weston 
machines  had  an  intensity  of  magnetism  of  18,000  to 
21,000  useful  lines  of  force  per  square  inch  of  pole  piece 
surface,  while  in  the  Edison  machines  this  intensity  was 
from  23,000  to  26,000;  these  higher  figures  are  probably 
due  to  wrought  iron  being  used  in  the  field.  In  cases 
where  the  field  is  as  economically  proportioned  and  as  well 
designed  as  in  these  machines,  an  intensity  equal  to  these 
figures  can  be  used  in  calculating  armatures;  but  if  the 
field  is  not  as  well  designed,  it  is  advisable  to  use  a  smaller 
intensity  in  calculating  armatures,  in  order  to  be  on  the 
safe  side. 

Another  important  proportion  in  designing  armatures  is 
the  size  or  cross-section  of  the  wire  used.  This  evidently 
depends  on  the  current  which  is  to  flow  through  the  ma- 
chine and  also  on  the  amount  of  electrical  energy  which  is 
to  be  lost  in  the  armature.  It  is  not  simply  dependent  on 
the  current,  but  is  governed  also  by  the  number  of  volts 
per  foot  which  are  to  be  generated  in  the  wire  ;  for  suppose, 
for  instance,  that  four  per  cent  of  the  total  electrical  en- 
ergy is  allowed  as  loss  in  the  armature,  which  will  be  a 
certain  number  of  volt-amperes  or  watts,  and  will  represent 
a  certain  definite  amount  of  heat  which  must  be  dissipated 


72  Principles  of  Dynamo-Electric  Machines. 

on  the  surface  of  the  armature  ;  from  the  constants  given 
above  for  the  induction  per  foot,  the  required  length  of 
wire  can  be  calculated  and  from  this  length  together  with 
the  volt-amperes  which  may  be  lost  in  it,  the  resistance  and 
cross-section  of  the  wire  can  readily  be  calculated.  Now  it 
is  evident  that  if  by  any  means  this  induction  per  foot  of 
wire  may  be  increased,  for  instance  doubled,  the  length  of 
the  wire  may  be  halved,  and  therefore,  to  have  the  same 
resistance,  the  cross-section  may  also  be  halved,  which  will 
decrease  the  amount  of  wire  by  weight  to  one-quarter  of 
what  it  was  before  ;  now  if  the  size  of  the  cooling  surface 
of  the  armature  remains  the  same,  that  is,  if  the  number  of 
layers  have  been  reduced  to  about  one-third  of  what  they 
were  before,  the  size  of  the  armature  remaining  about  the 
same,  the  armature  will  not  get  any  hotter,  as  the  amount  of 
heat  generated  is  the  same  and  the  cooling  surface  the  same. 
The  cross-section  of  the  wire  for  carrying  the  same  current 
has,  therefore,  been  reduced  to  one-half  of  what  it  was  be- 
fore, without  heating  the  armature  more.  This  calculation 
will  be  modified  somewhat,  on  account  of  the  resistance  of 
the  dead  wire  in  the  armature,  which  was  not  considered 
in  the  above  statement  in  order  to  avoid  complication  of 
the  calculation.  The  great  importance  of  increasing  as 
much  as  possible  the  number  of  volts  per  foot  is  seen  from 
the  example  just  given,  in  which  it  was  shown  that  in 
doubling  the  induction  per  foot,  by  increasing  the  speed  or 
the  intensity  of  the  field,  the  amount  of  active  wire,  by 
weight,  is  reduced  to  one-quarter  of  what  it  was,  for  the 
same  loss  in  the  armature  and  the  same  current.  This  also 
represents  a  decreased  self  induction  (provided  the  speed 
is  the  same),  less  sparking  at  the  commutator,  less  counter 
magnetism  of  the  armature,  and  less  shifting  of  the  brushes, 
provided  the  outside  dimensions  of  the  armature  remain 
approximately  the  same.  There  will  also  be  an  increased 
intensity  of  the  field,  as  the  space  between  the  pole  pieces 
and  the  armature  core  has  been  reduced  to  about  one- 
third  of  what  it  was. 


Armatures.  73 

In  order  to  serve  as  a  guide  in  preliminary  calculations 
of  armatures,  certain  values  for  the  proportion  of  the  cross- 
section  of  the  wire  and  the  current  are  sometimes  given. 
A  common  rule  is  to  use  about  three  amperes  per  square 
millimetre  cross-section,  in  which  it  must  be  remembered 
that  only  half  of  the  current  flows  through  a  wire  on  the 
armature,  because  the  two  halves  are  in  multiple  arc.  This, 
reduced  to  a  more  practical  form  in  our  own  units,  is  equiva- 
lent to  520  square  mils  per  ampere.  In  the  Weston  and 
Edison  machines  mentioned  above,  the  following  propor- 
tions exist  :  in  the  Weston,  from  375  to  562  square  mils  per 
ampere;  and  in  the  Edison,  from  400  to  500,  where  a  single 
wire  is  used;  and  475  to  600  for  the  sum  of  the  cross-sections 
where  two  and  three  wires  were  used  in  multiple  arc.  These 
figures  being  taken  from  the  same  machines  from  which  the 
constants  given  above  for  the  induction  per  foot  were  cal- 
culated, they  may  be  used  in  armatures  in  which  the  induc- 
tion is  about  the  same  as  in  these  machines. 

Another  important  point  in  designing  armatures  is  the 
thickness  of  the  space  occupied  by  the  wire  on  the  arma- 
ture. As  this  depends  on  so  many  of  the  other  proportions, 
such  as  the  cross-section  and  length  of  wire,  the  diameter 
and  length  of  the  -armature,  the  induction  per  foot,  the 
speed,  etc.,  it  is  hardly  possible  to  give  any  definite  rule. 
The  general  rule,  already  given  in  a  previous  chapter,  is  to 
make  it  as  small  as  possible.  The  following  constants 
taken  from  the  Weston  and  Edison  machines  may  serve  as 
a  guide.  The  percentage  of  the  external  diameter  of  the 
armature  which  is  taken  up  by  the  windings  on  both  sides 
(or  in  other  words  the  double  depth  of  the  winding  divided 
by  the  external  diameter)  was  as  follows  :  in  the  Weston 
machines  8  to  10  per  cent,  and  in  the  Edison,  8.8  to  11.5 
per  cent.  ;  that  is,  in  a  Weston  armature  of  say  1 0  inches 
external  diameter,  the  wire  would  occupy  from  .8  to  1  inch, 
making  the  thickness  of  the  layers  from  .4  to  one-half  inch. 
Possibly  this  depth  may  be  increased  without  decrease  of 
effect,  if  iron  lugs  or  projections  are  placed  between  the 


74  Principles  of  Dynamo-Electric  Machines. 

coils  as  in  the  Pacinotti  ring  armature,  but  this  point  we 
believe  has  not  yet  been  conclusively  demonstrated. 

The  distance  between  the  pole  piece  projections  where 
they  are  nearest  to  each  other,  depends  on  the  distance 
between  a  pole  piece  and  the  iron  core  of  the  armature, 
that  is,  on  the  depth  of  the  armature  windings  and  the 
clearance  between  the  armature  and  the  pole  pieces  ;  for  it 
is  evident  that  the  lines  of  force  at  this  part  of  the  field 
have  the  choice  of  two  paths,  either  from  one  pole  piece  to 
the  core  and  then  from  the  core  to  the  other  pole  piece,  or 
else  directly  from  one  pole  piece  to  the  other  ;  in  the  first 
case  they  are  rendered  useful  as  they  are  cut  twice  by  the 
armature  wire,  while  in  the  latter  case  they  are  wasted,  and 
represent  leakage  of  magnetism.  Suppose  the  distance 
between  the  pole  piece  projections  was  equal  to  twice  the 
depth  of  the  winding  and  the  clearance,  then  the  intensity 
of  the  field  between  them  would  be  as  great  as  the  useful 
field  between  the  pole  pieces  and  the  armature  core  at  that 
place,  and  would,  therefore,  represent  a  considerable  waste. 
The  amount  of  this  leakage  would  be  dependent  on  the 
amount  of  surface  exposed  on  the  ends  of  these  pole  piece 
projections.  The  distance  between  the  pole  piece  pro- 
jections should  be  made  as  many  times  the  depth  of  the 
windings  as  possible,  not  only  on  account  of  the  leakage, 
but  also  to  make  the  field  as  weak  as  possible,  for  the 
armature  coils  lying  between  these  pole  piece  projections, 
as  these  coils  are  the  ones  which  are  short  circuited  by  the 
brushes,  and  should  be  as  free  from  induction  as  possible. 
To  make  this  distance  too  great  would  diminish  the  amount 
of  active  surface  of  the  armature  too  much.  Perhaps  the 
best  guide  is,  to  make  this  distance  a  certain  number  of 
times  the  distance  between  the  pole  piece  and  the  iron 
armature  core  ;  about  seven  to  eight  times  is  a  fair  pro- 
portion. In  the  Weston  machines  it  was  found  to  be  about 
4.75  to  5.75  times,  while  in  the  Edison  it  was  from  4.4  to 
8  times.  By  making  it  great,  say  eight  to  nine  times, 
the  brushes,  for  an  armature  with  comparatively  few 


Armatures.  75 

windings,  will  require  less,  if  any,  adjustment  for  different 
loads,  as  the  field  is  weakened  in  the  place  where  the  dead 
(short  circuited)  coils  are. 

Another  rule  for  this  proportion  is,  to  make  the  distance 
between  two  pole  piece  projections  about  10  to  12  per  cent, 
of  the  whole  armature  circumference  ;  this  will  make  the 
active  surface  of  the  armature  about  80  to  76  per  cent,  of 
the  whole  surface,  a  figure  which  is  very  convenient  to  use 
in  preliminary  calculations  of  armatures.  This  is  about 
the  proportion  which  exists  in  the  Edison  and  Weston 
machines.  In  a  certain  100  incandescent  light  compound- 
wound  machine  of  100  volts,  in  which  the  distance  between 
two  pole  piece  projections,  was  about  14  per  cent,  of  the 
whole  circumference,  the  sparking  was  almost  completely 
avoided,  and  the  brushes  required  no  adjusting  for  differ- 
ent loads  ;  it  was  impossible  to  see  any  increase  of  spark- 
ing when  the  whole  load  of  100  lamps  was  suddenly  cut 
out  or  put  into  circuit.  This  shows  the  importance  of 
allowing  sufficient  space  between  the  ends  of  the  pole  piece 
projections. 


CHAPTER    V. 

Armatures.  —  (  Continued.) 

IN  winding  a  Gramme  armature,  the  following  are 
among  the  most  important  points  to  be  observed.  The 
available  space  for  the  wire  should  be  utilized  as  complete- 
ly as  possible,  and  therefore  should  contain  no  wooden  or 
non-magnetic  lugs  or  partition  pieces,  and  as  little  as  pos- 
sible of  the  frame  work  necessary  to  hold  the  armature  to 
the  shaft,  and  no  more  insulating  material  than  is  necessary. 
The  length  of  the  wire  should  be  as  small  as  possible  for 
the  required  number  of  windings,  and  therefore  should 
be  wound  as  closely  as  possible.  The  wire  should  be  so 
wound  as  not  to  be  liable  to  slip,  one  turn  over  another,  or 
to  change  position  after  completion,  as  this  loosens  the  wind- 
ing, thereby  causing  abrasion  and  a  resulting  short  circuiting 
in  the  armature.  The  successive  layers  should  therefore,  pre- 
ferably, when  practicable,  be  wound,  as  shown  in  figure  12, 
rather  than  as  shown  in  figure  13,  for  in  the  latter  case  they 


.  1.Q  iKig.  13 


may  slip  into  the  position  in  figure  1 2,  thus  loosening  the  wire 
and  thereby  decreasing  the  external  diameter  of  the  arm  ature ; 
although  this  may  be  very  little,  yet  in  many  cases  it  might 
be  j  ust  enough  to  loosen  the  tie  bands  around  the  outside  of  it, 
which  may  result  in  serious  consequences.  It  is  preferable, 
particularly  in  high  potential  machines,  that  the  wires  at  the 
beginning  and  end  of  the  same  coil  do  not  cross  each  other 
where  they  are  tightly  pressed  together,  unless  carefully  in- 
sulated, because  where  two  single  wires  cross  each  other  they 

(76) 


Armatures.  77 

touch  in  only  one  point,  and  it  requires  but  a  very  slight 
amount  of  abrasion,  or  a  slight  blow,  to  crush  the  insulation 
and  make  contact,  thus  short  circuiting  that  coil,  and  in  many 
cases  burning  off  the  wires.  Even  if  the  insulation  is  not 
ruptured  in  such  cases,  a  lightning  discharge  through  the 
armature  from  an  air  line  circuit,  or  a  high  self-induction 
spark,  such  as  might  occur  when  an  accidental  short  circuit 
was  suddenly  broken,  might  "jump"  through  the  insula- 
tion, thus  starting  an  arc  which  the  current  of  the  machine 
would  then  maintain.  An  armature  with  few  commutator 
bars  and  few  coils  of  many  turns  is  more  liable  to  such  an 
accident  than  one  in  which  the  windings  per  coil  are  few, 
and  the  electromotive  force  per  coil  correspondingly  small. 
In  winding  a  Gramme  armature  in  the  ordinary  way  the 
wire  from  the  beginning  of  the  lowest  layer  has  to  pass 
to  the  outside  between  the  coils,  which  in  a  well  and 
closely  wound  armature  will  cause  some  irregularity  in  the 
winding  owing  to  the  space  it  occupies,  particularly  as  it 
should  have  an  extra  insulation  with  tape.  Furthermore, 
if  this  end  breaks  off  short,  as  it  is  apt  to  do,  the  whole 
coil  must  be  unwound.  Both  of  these  objections  are  over- 
come, while  at  the  same  time  other  advantages  are  gained 
by  the  following  simple  and  ingenious  winding  which  we 
believe  is  now  largely  used.  Find  the  actual  length  of 
Avire  required  for  one  coil,  by  winding  it  temporarily,  cutting 
it  to  the  right  length,  and  then  unwinding  it.  Cut  the  others 
to  this  length.  Find  the  middle  of  the  length  of  the 
wire  for  one  coil,  for  instance  by  doubling  it  on  itself,  and 
mark  this  middle  point.  Start  the  winding  by  placing  this 
middle  point  of  the  wire  in  the  middle  position  of  the  first 
layer,  as  at  M,  figure  14,  and  having  clamped  or  tied  it,  start 
with  either  end  and  continue  winding  just  as  if  the  middle 
point  was  the  beginning  of  the  coil,  neglecting  for  the  time 
the  other  half  of  the  wire.  The  space  for  this  first  half  of  the 
coil  when  wound,  must,  however,  be  only  half  the  width  of 
the  space  for  the  whole  completed  coil.  Having  thus 
wound  the  first  half,  take  the  rest  of  the  wire  on  the 


78 


Principles  of  Dynamo-Electric  Machines. 


other  half  of  the  middle,  and  wind  it  in  the  same  way 
in  the  remaining  half  space  for  that  coil,  thus  bring- 
ing both  ends  of  the  wire,  that  is,  both  the  beginning 
and  end  of  the  finished  coil,  on  the  outside  layer.  If  there 
is  an  odd  number  of  layers,  these  ends  will  be  at  the  two 
ends  of  the  outside  or  last  layer,  thus  being  in  the  best 
possible  position  to  be  connected  to  the  end  of  the  previous 
coil  and  the  beginning  of  the  next,  respectively.  As  this 
brings  both  ends  of  each  coil  on  the  outside  layer,  it  over- 
comes some  of  the  difficulties  in  winding  a  smooth  com- 
pact armature,  and  has  the  great  advantage  that  there  is  no 
irregularity  in  the  winding,  nor  is  there  danger  of  short 


:  14 


circuiting  caused  by  the  beginning  of  the  wire  passing  out 
between  the  others  from  the  lowest  layer  to  the  outside.  It 
also  has  the  advantage  that  in  case  the  end  breaks  off  it  is 
easily  spliced  as  it  can  readily  be  unwound  for  one  or  two 
turns.  In  practice  when  there  are  more  than  three  layers  it 
is  preferable  to  wind  the  half  layers  successively,  and  alter- 
nately with  each  end  of  the  wire,  as  illustrated  in  figure  14, 
thus  completing  each  full  layer  before  starting  the  next.  If 


Armatures.  79 

the  number  of  layers  is  even,  the  two  ends  of  a  completed 
coil  will  both  be  in  the  middle  of  the  outside  layer  ;  this, 
however,  is  not  objectionable. 

When  iron  lugs  are  used  between  the  coils,  it  is  prefer- 
able to  make  them  wedge-shaped  so  that  the  sides  of  the 
space  for  one  coil  are  parallel,  as  this  facilitates  the  wind- 
ing and  makes  it  smoother.  If  there  are  no  such  lugs  the 
coil  space  itself  is  narrower  inside  the  ring  than  outside  and 
there  will  be  more  layers  on  the  inside  than  on  the  outside  ; 
the  winding  will,  therefore,  not  be  quite  as  smooth  at  the 
curved  ends  but  can  be  made  to  appear  compact  and 
smooth  by  placing  a  piece  of  cardboard  under  the  last  layer. 
Every  dynamo  builder  takes  pride  in  having  handsome 
looking  armatures,  especially  as  a  carelessly  and  loosely 
wound  one  carries  with  it  the  impression  that  the  whole 
machine  is  built  after  the  same  fashion. 

In  order  to  make  the  winding  smooth  in  cases  in  which  no 
lugs  are  used,  resort  is  often  had  to  the  device  of  placing 
strips  of  thick  card  board  in  the  empty  space  in  each  layer 
on  the  outside  of  the  ring,  when  the  corresponding  space 
on  the  inside  is  full,  in  order  to  make  them  come  out  even. 
But  in  cases  of  armatures  of  small  diameter  and  many 
layers,  this  wastes  valuable  space  and  had  therefore  better 
not  be  done. 

Prior  to  winding  a  Gramme  armature  the  spaces  for  the 
coils  should  be  accurately  and  carefully  marked  off  in  pen- 
cil on  the  outside  of  the  core  after  it  is  thoroughly  insu- 
lated with  several  layers  of  shellaced  paper  or  muslin.  It 
is  bad  practice  to  wind  the  coils  without  spacing  them  off 
properly  and  to  let  the  last  coil  take  up  any  difference,  for, 
as  pointed  out  before,  it  is  very  necessary  that  all  the  coils 
should  be  alike  in  every  respect. 

When  there  are  no  iron  lugs  to  serve  as  guides,  it  will  be 
found  greatly  to  facilitate  the  winding  to  make  two 
wooden  clamps  as  shown  in  figure  15,  the  oblong  hole  in 
the  inside  of  which  fits  the  core  tightly,  one  side  of  each 
of  the  clamps  being  radial  to  the  centre  of  the  core.  These 


80 


Principles  of  Dynamo-Electric  Machines. 


are  clamped  securely  to  the  core  at  the  pencil  marks  of  the 
space  for  a  coil  and  serve  to  keep  the  coil  in  place  while 
winding  it.  Two  or  three  pieces  of  tape  may  be  laid  trans- 
versely in  the  troughs  thus  formed  for  the  coil,  and  after- 
ward closed  over  the  outside  of  the  coil  when  completed, 
thereby  keeping  it  from  collapsing  when  the  wooden 
clamps  are  removed. 

The  end  of  one  coil  and  the  beginning  of  the  next  being 
fastened  to  the  same  commutator  bar,  the  current  in  these 
coils  has  to  pass  through  one  of  these,  through  the  clamped 
or  soldered  contact  at  the  commutator  bar,  and  back 
through  the  other  wire,  in  all  of  the  connections  except  at 
the  two  or  four  where  it  flows  off  to  the  brushes.  Although 


ig.  15 


the  resistance  of  each  of  these  short  lengths  of  wire,  and 
of  the  soldered  or  clamped  contacts,  may  be  small,  yet 
there  are  many  of  these  resistances  in  series,  and  they  may, 
therefore,  form  an  appreciable  part  of  a  low  resistance 
armature,  especially  when  the  contact  with  the  commutator 
bars  is  not  very  good.  It  is  therefore  preferable  to  strip 
the  insulation  for  a  short  distance  on  these  two  wires 
quite  close  to  the  coil,  and  either  twist  or  bind  them  to- 
gether and  afterwards  solder  them.  This  is  especially  to 
be  recommended  in  cases  where  the  commutator  is  some 


Armatures.  81 

distance  from  the  armature,  as  for  instance  when  it  is  out- 
side of  the  bearing. 

Binding  wires  on  the  outside  of  the  armature  should  be 
tightly  wound  and  very  well  insulated  from  the  wires,  pre- 
ferably with  mica.  It  is  better  to  make  them  narrow  and  at 
frequent  intervals  rather  than  broad  and  fewer  of  them,  as 
currents  are  induced  in  them  as  well,  and  if  broad, 
these  currents  have  a  chance  to  circulate  through  their  very 
low  resistance,  thus  heating  them.  If,  as  is  usual,  fine  hard 
brass  wire  is  used,  all  the  wires  of  one  band  should  be 
soldered  together  to  avoid  an  uncoiling  in  case  one  of 
the  wires  is  broken.  Iron  or  steel  tie  wires  should  in 
no  case  be  used  as  it  would  partially  shield  the  core  from 
magnetization. 

Double  covered  wire  only  should  be  used  for  armatures. 
The  windings  should  be  shellaced,  preferably  with  very 
thin  shellac,  while  they  are  being  wound.  Armatures  are  fre- 
quently baked  for  a  day  to  dry  the  shellac,  especially  when 
the  winding  is  very  open,  as  in  spherical  or  carelessly 
wound  armatures. 

Iron  wire  has  been  suggested  to  replace  the  copper  con- 
ductors, as  it  acts  both  as  an  iron  core  and  as  the  conductor. 
But  as  the  resistance  is  so  much  greater  (about  6  times) 
it  does  not  seem  to  have  been  a  success  in  practice.  Copper 
coated  iron  wire  and  copper  wire  covered  with  a  thin 
layer  of  iron,  have  been  suggested  and  we  believe  have  been 
tried  successfully. 

The  laminating  of  the  core  has  already  been  described. 
It  was  shown  that  the  best  form  was  to  make  the  core  of 
thin  discs  separated  by  thin  paper  ;  but  they  are  also 
frequently  made  of  iron  wire,  which  is  first  rusted  to 
insulate  it.  The  former  is  preferable,  as  the  iron  circuit 
of  the  lines  of  force  in  the  core  is  then  continuous,  as 
distinguished  from  the  course  transversely  to  a  bundle 
of  round  iron  wires.  In  all  cases  the  core  as  a  whole 
should  form  a  compact  rigid  mass  ;  there  should  be  no 
possibility  of  any  parts  of  it  becoming  loose.  Cast  iron 


82  Principles  of  Dynamo-Electric  Machines. 

armature  cores,  even  when  cut  with  slits  are  bad.  The 
iron  is  always  much  inferior  to  the  soft  wrought  iron  of 
plates  or  wires  ;  it  is  often  harder  ;  it  increases  the  length 
and  resistance  of  the  wire  as  it  is  necessary  to  have  a  larger 
bulk  of  iron  to  produce  the  same  magnetic  effect,  especially 
as  the  cuts  are  necessarily  quite  thick  ;  it  is  apt  to  break, 
and  if  made  at  all  well  it  is  probably  but  little  cheaper  than 
wrought  iron.  A  notable  illustration  is  in  the  Brush  machine 
which  formerly  had  a  cast  iron  armature  core  which  is 
replaced  in  the  present  machines  by  a  wrought  iron  one, 
increasing  the  capacity  of  the  machine,  we  are  informed, 
from  40  to  60  arc  lights,  or  50  per  cent.  The  wrought 
iron  armature  is  said  in  this  case  to  be  no  more  expensive 
than  the  cast  iron  one. 

It  is  best  not  to  trust  too  much  to  friction  to  securing  the 
armature  to  the  shaft,  but  to  fasten  it  by  some  more  reli- 
able method.  A  very  small  amount  of  slip  may  do  a  great 
amount  of  mischief.  It  must  be  remembered  that  the  whole 
horse-power  driving  the  dynamo,  acts  to  push  the  wires  of 
the  armature  over  the  core  in  a  cylinder  armature,  and 
to  twist  the  core  and  windings  off  the  shaft  in  a  Gramme 
armature.  The  force  to  displace  the  wires  is  precisely 
the  same  in  amount  as  if  a  brake  band  were  gripped 
so  tightly  around  the  outside  of  the  armature  as  to  cause 
it  to  require  the  same  force  to  turn  the  armature  as  when 
running  in  the  field  with  its  full  load.  When  the  arma- 
ture is  suddenly  short  circuited  the  sudden  increase  in  this 
force  to  tear  off  the  wires  amounts  almost  to  that  of  a 
blow  from  a  hammer. 

The  core  should  have  all  edges  rounded  off  smoothly,  and 
should  be  thoroughly  insulated  with  muslin,  tape  or  paper, 
well  shellaced. 

The  subject  of  the  ventilation  of  armatures  has,  appar- 
ently, occupied  much  more  attention  among  dynamo  build- 
ers, than  its  importance  seems  to  demand.  The  construction 
of  armatures  has  been  in  many  cases  greatly  complicated 
in  order  to  convert  the  internal  parts  into  fans  and  flues 


Armatures.  83 

for  ventilating  ;  in  one  case  the  armature  core  itself  was  a 
sort  of  centrifugal  blower  sucking  air  in  at  the  shaft  and 
driving  it  out  at  the  periphery.  But  to  the  technical  engi- 
neer it  cannot  fail  to  be  evident  that  it  is  much  better  to 
prevent  the  generation  of  heat  in  the  armature  by  proper 
construction,  than  to  sacrifice  the  efficiency  by  developing 
a  large  amount  of  heat,  and  then  still  further  reducing  the 
efficiency  by  using  part  of  the  power  to  mechanically  dis- 
sipate this  heat  by  forcing  air  through  the  armature.  There 
will  of  course  be  some  heat  generated  in  the  wire  and  in 
the  core  ;  but  by  proper  proportioning  of  the  cross  sec- 
tion of  the  wire,  by  proper  lamination  and  insulation  of  the 
core,  and  by  using  soft  wrought  iron,  this  amount  of  heat 
may  be  made  comparatively  small  ;  by  increasing  the 
diameter  of  the  armature  to  present  a  large  external  surface, 
this  heat  can  readily  be  dissipated  at  such  a  rate  that  the 
temperature  of  the  armature  remains  low  enough  not  to  be 
objectionable.  There  is  no  objection,  therefore,  to  making 
the  core  of  the  armature  compact,  without  flues  or  vents, 
provided  it  is  proportioned  so  as  not  to  heat  too  much. 
Practice  has  shown  that  in  armatures  without  internal  ven- 
tilation the  resistance  of  the  wire  may  be  so  proportioned 
that  it  absorbs  from  3  to  4  or  even  5  per  cent,  of  the  total 
electrical  energy. 

Commutators  should  be  made  with  as  many  bars  as  prac- 
ticable for  reasons  mentioned  before,  as  the  number  of 
coils  is  the  same  as  the  number  of  commutator  bars.  The 
absolute  number  of  bars  depends  on  numerous  proportions 
of  the  armature,  and  must,  therefore,  be  determined  in  each 
case.  As  a  guide  may  be  taken  the  results  of  good  engi- 
neering practice,  which  show  that  in  incandescent  light 
machines  of  somewhat  over  100  volts  the  number  of  com- 
mutator bars  is  so  proportioned  that  the  mean  number  of 
volts  between  two  bars,  that  is,  the  induction  in  one  coil, 
is  from  4  to  7  volts.  In  other  words,  if  the  machine  is  to 
give  120  volts,  and  it  is  decided  to  assume  about  5  volts 
per  commutator  bar,  and  if  80  per  cent,  of  the  wires  are 


84  Principles  of  Dynamo-Electric  Machines. 

active,  there  will  be  120  — r-  5  =  24  active  bars  on  one- 
half,  and  as  this  is  80  per  cent.,  the  total  number  in  one-half 
would  be  24  -r-  .80  =  30,  making  about  60  bars  in  all.  In 
high  potential  arc  light  machines  the  number  of  volts  per 
commutator  bar  must  necessarily  be  taken  greater  as  it 
would  otherwise  make  the  number  of  bars  too  great.  The 
objection  to  a  large  number  of  volts  per  coil  is  not  so  great 
in  arc  light  machines,  as  the  current  is  always  small  in  com- 
parison to  incandescent  light  machines,  and  the  damage 
which  a  spark  at  the  brushes  causes  decreases  with  the 
current.  The  long  bright  spark  of  a  high  tension  and  low 
current  machine  may  not  destroy  the  commutator  nearly 
as  quickly  as  the  less  bright  spark  of  a  machine  giving  a 
large  current.  Twenty  volts  is  said  to  be  the  lowest  potential 
that  will  maintain  an  arc  ;  it  is  therefore  preferable,  if 
not  almost  essential,  that  the  maximum  volts  per  commu- 
tator bar  should  be  less  than  20,  at  least  for  those  near  the 
brushes,  otherwise  an  arc  between  two  bars  may  be  estab- 
lished by  the  brushes  which  then  continues  to  burn, 
causing  the  well-known  flash  encircling  the  whole 
commutator. 

The  thickness  of  the  insulation  between  the  bars  should 
not  be  less  than  one  fiftieth  of  an  inch,  as  the  danger  of 
the  "  jumping  "  of  the  self-induction  spark  becomes  too 
great  if  the  distance  is  much  less.  In  a  case  in  which  this 
was  less  the  commutator  was  seen  to  be  covered  with 
myriads  of  small  sparks,  which,  although  doing  little  harm 
themselves,  are  too  apt  to  establish  more  dangerous  arcs. 
The  material  used  for  insulation  should  not  be  anything 
that  will  char,  thereby  being  converted  into  conducting 
carbon  ;  it  should  not  be  gritty  as  it  then  acts  to  wear  off 
both  the  brush  and  the  commutator  ;  it  should  not  wear 
off  less  rapidly  than  the  bars,  as  it  would  cause  the  brushes 
to  vibrate,  causing  sparking,  and  an  objectionable  humming 
noise.  Air  insulation  between  the  bars  is  not  good, 
except  where  the  space  can  be  made  very  large  as  in  the 
Brush  and  Thomson-Houston  machines,  as  it  is  too  apt  to 


Armatures.  85 

fill  with  copper  dust  or  other  conducting  material,  making 
contact  between  them. 

The  connection  of  the  coils  to  the  commutator  bars  are 
either  soldered  or  else  clamped  with  screws.  The  objec- 
tion to  the  former  is,  that  the  whole  armature  has  to  be  re- 
turned to  the  makers  to  have  the  commutator  renewed, 
while  the  objection  to  the  latter  is,  that  the  screws  do  not 
make  such  a  good  contact,  and  are  apt  to  become  loose  and . 
open  the  circuit.  The  choice  between  them  is  the  choice 
between  two  evils.  If  the  wire  terminals  are  in  the  form 
of  flattened  loops,  and  if  the  screws  are  frequently  over- 
hauled, the  latter  is  probably  the  better  method.  Rosin 
should  always  be  used  as  a  flux  in  soldering,  in  preference 
to  acid,  except  when  used  by  skillful  and  careful  persons, 
as  a  drop  of  acid  on  the  coil  or  core  may  do  considerable 
damage  in  course  of  time. 

For  single  machines,  a  commutator  is  easily  made  by 
casting  it  as  a  massive  cylinder,  turning  it  off  to  fit.  the 
holder  and  then  cutting  it  into  strips  on  a  gear-cutting 
machine.  These  strips  are  then  insulated  and  fastened  to- 
gether in  the  usual  way  with  conically  surfaced  rings. 

In  order  to  bring  the  brushes,  or  the  line  of  commutation 
into  a  convenient  position,  the  connections  to  the  commuta- 
tor may  be  made  at  an  angle,  as  if  the  commutator  had  been 
twisted  at  an  angle  with  reference  to  the  armature,  after  the 
connections  were  made.  This  is  often  resorted  to  as  one  of 
the  "  tricks  of  the  trade,"  to  make  it  appear  as  if  the  neutral 
line  was  exactly  perpendicular  to  the  axis  of  magnetization. 

The  brushes  should  not  be  made  of  wire  alone,  as  they 
are  too  apt  to  cut  grooves  into  the  commutator,  therefore 
necessitating  frequent  dressing  of  its  surface.  Nor  should 
they  be  of  solid  sheets  if  they  are  broad,  as  they  then  do 
not  always  make  good  contact  along  the  whole  width. 
They  should  be  cut  lengthwise  into  narrow  strips,  and 
should,  for  large  current  machines,  be  quite  thick.  A 
good  brush  for  incandescent  machines,  may  be  made  with 
alternate  layers  of  wire  and  sheets  with  longitudinal  cuts 


86  Principles  of  Dynamo-Electric  Machines. 

in  it.  A  brush  should  not  at  any  time  touch  more  than 
two  bars,  except  when  there  are  very  many  commutator 
bars,  and  when  the  distance  between  the  pole-piece  pro- 
jections, or  the  width  of  the  neutral  field,  is  very  large. 
It  is  a  mistake  to  think  that  when  the  brushes  are  pressed 
down  very  hard  the  sparking  is  diminished  ;  on  the  con- 
trary, this  often  increases  the  sparking.  With  well  fitting 
brushes  and  a  smooth  commutator,  it  will  suffice  to  have 
them  touch  very  lightly  only,  provided  they  do  not  vibrate 
so  as  to  leave  the  surface. 


CHAPTER   V. 

Armatures. — ( Concluded.) 

CYLINDER  armatures  differ  from  the  Gramme  principally 
in  the  following  respects.  Their  advantages  over  the 
latter  are  :  for  the  same  magnetism  in  the  field  the  volume 
of  the  armature  space,  and,  therefore,  also  the  size  of  the  de- 
pendent parts  of  the  rest  of  the  machine,  is  smaller,  owing 
to  the  lost  air  space  in  the  inside  of  the  Gramme  ring ;  the 
proportion  of  the  wire  on  the  armature  which  is  active,  is 
greater,  that  is,  for  the  same  field,  electromotive  force,  cur- 
rent, and  inductor  velocity,  the  amount  of  wire  required 
on  the  armature  is  less ;  for  the  same  capacity  of 
armature,  the  internal  resistance  is  smaller ;  when  thick 
wire  is  used  for  the  windings,  it  is  more  easily  handled 
than  in  the  Gramme  ;  it  is  easier  to  fasten  the  core  rigidly 
to  the  shaft,  without  the  loss  of  valuable  wire  space  around 
the  core  ;  they  are  more  easily  centered  or  balanced  ac- 
curately for  high  speeds  ;  the  core,  if  made  of  sheet  metal, 
is  less  expensive,  as  there  is  less  waste  ;  an  unbalanced 
field,  that  is,  one  in  which  leakage  or  other  causes  make  one 
pole  stronger  than  the  other,  will  affect  it  less  than  it  would 
a  Gramme  ring. 

The  disadvantages  are  :  it  is  more  difficult  to  insulate 
the  wires  for  high  potentials,  as  two  neighboring  wires 
may  have  the  whole  difference  of  potential  of  the  machine 
in  them  ;  a  high  inductor  velocity  is  not  so  easily  obtained 
without  great  increase  of  weight  of  armature,  as  its  weight 
increases  with  the  square  of  the  diameter,  while  in  the 
Gramme  it  increases  merely  as  the  diameter  ;  if  one  coil 
burns  out  it  will  often  necessitate  the  unwinding  of  several 
others,  or  even  of  all  the  others,  as  the  injured  one  is 
frequently  among  the  first  or  inside  coils  ;  it  is  almost 

(87) 


88  Principles  of  Dynamo-Electric  Machines. 

impossible  to  have  all  the  coils  symmetrically  situated  and 
of  the  same  length  ;  for  the  same  cross-section  of  armature 
core,  it  presents  less  external  surface  for  the  induction 
wires  ;  it  is  more  complicated  to  wind  properly,  and  there- 
fore requires  a  more  skilled  mechanic. 

In  designing  cylinder  armatures  the  following  points  are 
the  most  important,  besides  those  already  given.  The 
greater  the  length  as  compared  to  the  diameter  the  greater 
will  be  the  proportion  of  active  wire.  On  the  other  hand, 
the  greater  the  diameter  the  higher  will  be  the  inductor 
velocity,  which  it  is  desirable  to  make  as  great  as  prac- 
ticable. Both  diameter  and  length  should  therefore  be 
made  as  great  as  practicable  ;  but  as  a  practical  limit  is 
soon  reached,  it  is  necessary  to  determine  by  trial  calcula- 
tions for  each  special  case,  which  of  the  two  advantages 
has  the  greater  weight.  For  high  potential  machines  in 
which  a  slightly  greater  amount  of  dead  resistance  is  not 
very  objectionable,  the  diameter  might  be  increased,  while 
for  low  potential,  quantity  machines,  the  amount  of  dead 
resistance  is  of  more  importance,  and  therefore  it  might  be 
better  to  increase  the  length  of  the  core.  As  other  con- 
siderations may  however  have  greater  weight  in  special 
cases,  no  general  rule  can  be  given. 

The  coils  should  all  have  as  nearly  as  possible  the  same 
length  and  resistance,  and  should  all  be  symmetrically 
situated  with  respect  to  the  field  and  to  their  mean 
distance  from  the  centre  of  the  shaft.  The  difficulty  of  ac- 
complishing this  is  comparatively  great,  and  it  is  therefore 
of  the  utmost  importance  to  guard  against  irregularities  of 
winding,  for,  as  stated  before,  if  the  electromotive  force 
induced  in  one  half  of  the  armature  is  greater  than  that  in 
the  other,  even  if  only  a  small  amount,  a  current  will  cir^ 
culate,  on  open  circuit,  in  the  armature  wire  itself,  which 
by  Ohm's  law  is  equal  to  this  difference  of  electromotive 
force  divided  by  the  resistance  of  the  wire,  and  as  the 
latter  may  be  very  small,  this  wasted  current  may  be  quite 
great.  On  the  other  hand,  if  the  inductions  in  the  two 


Armatures.  89 

halves  are  exactly  equal  and  the  resistances  unequal,  the 
differences  of  potential  at  the  ends  of  the  two  halves  (that 
is,  the  total  electromotive  forces,  less  that  absorbed  by  the 
armature  wire  itself)  will  be  unequal  when  the  machine  is 
running  with  a  load,  and  there  will  be  irregularities  or  pul- 
sations in  the  current  which  may  increase  the  sparking. 

A  coil  should  not  be  too  wide,  measured  along  the 
periphery  of  the  armature,  in  order  that  the  coil  which  is 
short  circuited  by  a  brush  may  be  entirely  in  the  neutral 
part  of  the  field.  The  real  neutral  field  is  always  con- 
siderably smaller  than  the  distance  between  the  pole  piece 
projections.  The  coils  should,  for  the  same  reason,  be 
wound  so  that  the  two  sides  are  as  nearly  as  possible 
diametrically  opposite  to  each  other,  though  they  need  not 
be  exactly  opposite. 

As  the  wires  have  to  cross  over  one  another  at  the  ends 
of  the  armature,  they  occupy  considerable  space  there,  and 
make  what  is  termed  the  "  heads."  It  is  evidently  well  to 
make  these  as  small  as  possible,  for  several  reasons  ;  it 
shortens  the  shaft  and  the  distance  between  the  bearings, 
thus  decreasing  the  width  of  the  machine  ;  it  shortens  the 
bearing  braces  or  supports  and  reduces  the  tendency  to 
vibration  as  the  shaft  is  thereby  stiffened ;  it  makes  the 
length  or  resistance  of  the  different  coils  more  nearly 
equal,  as  the  lengths  gradually  increase  from  the  lowest,  or 
first  coil,  to  the  last,  and  with  large  heads  this  increase  is 
evidently  considerable  ;  a  small  head  is  more  likely  to  be 
flat,  therefore  diminishing  the  tendency  of  the  wire  to  slip 
off  and  loosen  the  coil  ;  it  will  facilitate  making  the  wind- 
ings compact  and  solid  to  have  a  small  head.  In  order  to 
reduce  this  head,  the  number  of  windings  should  be  as 
small  as  practicable  ;  the  number  of  crossings  should  be  as 
few  as  possible,  and  the  turns  of  one  coil  should  therefore 
be  parallel  and  not  cross  over  one  another ;  the  wires  of 
the  head  should  be  bent  around  so  as  not  to  bring  several 
crossings  over  each  other,  that  is,  the  crossings  should  be 
distributed  over  the  head  so  as  to  keep  it  as  flat  as  possible. 


90 


Principles  of  Dynamo-Electric  Machines. 


When  wires  at  the  head  are  unavoidably  in  such  a  position 
as  to  be  likely  to  slip,  they  should  be  tied  with  strong 
twine  or  tape,  in  order  to  hold  them  securely  in  their 
places.  All  wires  crossing  each  other  should  be  particularly 
well  insulated  with  tape,  shellaced  muslin,  fibre,  or  dense 
shellaced  cardboard  which  is  not  likely  to  break. 

The  general  principle  of  the  cylinder  winding  is  as  fol- 
lows. If,  as  in  figure  16,  a  cylinder  be  supported  on  its 
axis,  and  while  turning  slowly,  a  wire  be  wound  over  it 


lengthwise,  it  will,  after  having  made  one  complete  revolu- 
tion, be  wound  with  the  simplest  form  of  a  cylinder  wind- 
ing. To  complete  it,  the  end  of  the  wire  must  be  con- 
nected with  the  beginning  to  make  it  an  endless  wire,  and 
the  beginning  of  each  turn,  or  number  of  turns  forming  a 
coil,  must  have  a  branch  wire  attached  to  it  for  connecting 
it  to  the  commutator,  as  shown  by  the  bold  lines. 

In  continuing  the  winding  shown  in  this  figure,  it  will 
be  found  that  after  a  few  more  turns  the  armature  will  be 
completely  covered  with  the  wire,  while  the  winding  has 
been  continued  for  only  one-half  of  a  revolution  of  the 
cylinder.  Furthermore,  the  commutator  branch  connec- 
tions will  be  found  to  be  along  only  half  the  circumference, 


Armatures.  91 

while  the  beginning  and  end  do  not  meet  and  cross,  but  are 
parallel  and  in  the  same  direction,  so  that  they  cannot  be 
joined  properly.  This  is  often  misleading  and  confusing 
to  any  one  winding  a  cylinder  armature  for  the  first  time, 
as  it  gives  the  impression  that  there  is  some  mistake.  It 
will  be  easily  understood  if  we  remember  that  the  winding 
must  be  continued  for  one  complete  revolution  of  the 
cylinder,  and  as  it  is  entirely  covered  after  half  a  revolu- 
tion, it  follows  that  in  the  second  half  revolution  the  wire 
will  have  to  be  wound  over  that  already  on  the  cylinder, 
thus  making  two  layers  over  the  whole  surface. 

This  simple  form  of  cylinder  winding  has  the  objection 
that  the  coils  of  the  second  half  are  longer  and  larger, 
having  a  greater  mean  radius  and,  therefore,  a  greater 
Velocity,  as  they  are  wound  over  the  others.  This,  as 
stated  before,  is  a  fault  which  should  be  avoided,  if  possi- 
ble. It  can  be  overcome  to  a  great  extent  by  dividing  the 
surface  of  the  armature  into  the  proper  number  of  sections, 
as  shown  in  figure  17,  and  winding  at  first  only  in  every 
alternate  section,  until  half  of  the  whole  number  of  coils 
are  wound,  as  shown  in  light  lines,  and  then  continuing  the 
winding  of  the  rest  of  the  coils  in  the  remaining  alternate 
sections,  as  shown  by  the  dark  lines.  All  the  coils  will 
then  be  symmetrically  situated,  will  have  the  same  mean 
radius,  and  more  nearly  the  same  length.  This  method 
will  answer  very  well  when  there  are  numerous  turns  in 
each  coil,  or  in  general  when  the  width  of  the  cross- 
section  of  a  coil  is  greater  than  its  depth,  as  shown  in 
figure  18,  in  which  the  dark  and  light  sections  represent 
the  neighboring  coils  which  belong  to  diametrically  op- 
posite commutator  bars,  as  shown  in  figure  17.  This  pro- 
portion of  the  sides  of  the  cross-section  will  generally  be 
found  to  exist  when  the  number  of  coils  or  commutator 
bars  is  small. 

In  the  better  class  of  machines  this  is  however  not 
generally  the  case,  as  for  instance,  when  there  are  two 
turns  per  coil,  for  then  the  depth  is  greater  than  the  width, 


92  Principles  of  Dynamo-Electric  Machines. 

as  shown  in  figure  19,  and  there  may  be  difficulty  in  keep- 
ing the  coils  from  collapsing  while  they  are  being  wound. 
The  method  shown  in  figure  20,  is  often  resorted  to  in 
such  cases,  but  it  is  not  to  be  recommended  except  when 
there  are  very  many  coils  or  commutator  bars,  as  they 
have  different  mean  radii  from  the  axis.  The  difficulties 
mentioned  have  all  been  overcome  by  the  ingenious  method 
devised  by  Weston,  and  shown  in  figure  21,  in  which  each 
coil  is  split  into  two  equal  parts,  which  lie  alternately  over 
and  under  the  corresponding  parts  of  the  other  coil  belong- 
ing to  the  diametrically  opposite  commutator  bar.  In  this 
method  the  turns  are  wound  in  the  order  as  indicated  by  the 
numbers,  either  as  in  the  first  section  or  as  in  the  second,  the 
latter  being  less  confusing  while  winding  and  the  former 
simpler  to  the  experienced  winder.  The  advantages  of  the 


Weston  system  over  the  others  increases  with  the  number 
of  turns  per  coil,  and  decreases  with  an  increase  in  the 
number  of  coils.  In  the  method  in  figure  19,  the  order  of 
the  winding  may  be  either  as  shown  by  the  numbers  in 
the  first  section,  or  by  those  in  the  second  ;  the  former  is 
less  confusing,  while  the  latter  is  simpler. 

Referring  to  figure  22,  which  shows  an  end  view  of  the 
unwound  armature  with  eight  sections,  each  for  two  coils 
belonging  to  opposite  commutator  bars,  if  we  start  at  the 
first  upper  commutator  bar,  and  wind  into  the  first  section 
a,  we  may  return  to  the  face  in  any  one  of  the  opposite 
half-sections  #,  c,  d,  e,f,  thence  to  the  next  bar  and  the 
next  but  one  half-section  as  shown.  By  returning  in  the 
diametrically  opposite  part  d  it  will  be  the  old  Siemens 
winding,  which,  as  will  be  seen  by  completing  the  winding, 


Armatures. 


93 


is  slightly  irregular,  i  By  returning  in  e,  or  c,  it  will  be  the 
Froehlich  or  Breguet,  which  are  practically  identical  ;  both 
are  quite  regular.  By  returning  in  b,  or/,  the  winding  be- 
comes very  irregular  and  should  not  be  used.  If  the 
number  of  coils  is  odd,  the  Froehlich  and  the  Breguet  sys- 
tems merge  into  one,  that  is,  the  half -sections  e  and  c  be- 
come one  and  the  same,  being  then  in  the  position  of  d, 
diametrically  opposite  to  a.  This  is  known  as  the  Edison 


system.  The  Weston  system  is  the  old  Siemens  improved 
by  splitting  the  coils  as  described.  The  writer's  system  is 
this  same  principle  applied  to  the  Froehlich,  and  is,  there- 
fore, quite  regular  and  symmetrical.  The  chief  advantage 
of  the  Froehlich  over  the  Siemens,  besides  its  regularity,  is, 
that  there  are  only  half  as  many  crossings  of  the  coils  on 
the  ends,  thus  making  smaller  heads. 

When  iron  lugs  or  projections  are  used  between  the 

1.  A  detailed  discussion  of  these  forms  of  windings  will  be  found  in  the 
ELECTRICIAN  AND  ELECTRICAL  ENGINEER,  March  1886,  p.  84.    See  Appendix  IV. 


94 


Principles  of  Dynamo- Electric  Machines. 


coils,  the  winding  becomes  less  difficult.  In  that  case  each 
layer  in  one  section  (which  may  embrace  from  one  to  five 
or  six  coils)  should  be  completed  before  the  next  layer  is 
wound,  thus  frequently  necessitating  a  number  of  coils 
being  wound  at  the  same  time  ;  or,  if  this  is  confusing,  a 
pair  of  wooden  blanks  should  be  made  of  exactly  the  width 
of  the  vacant  part  of  one  section,  and  be  fastened  there 
until  the  coil  is  completed  and  bound  with  tape. 


Fig.  33 


When  there  are  no  lugs  or  projections  of  the  core,  the 
surface  of  the  armature  should  be  carefully  spaced  off  into 
the  required  number  of  sections.  It  will  then  be  found 
convenient,  if  not  essential,  to  make  wooden  blanks  of  ex- 
actly the  width  and  shape  of  the  section  of  a  coil,  and 
clamp  them  as  shown  in  figure  23,  leaving  a  trough  of  the 
exact  width  of  one  coil.  In  this  case,  it  is  advisable  to 
wind  first  every  alternate  section  and  bind  the  finished 
coils  with  tape,  in  order  to  leave  vacant  spaces  for  insert- 
ing these  wooden  guiding  blanks.  When  the  armature  is 


Armatures.  95 

half  wound  it  will  present  the  appearance  of  a  cogwheel, 
with  a  trough  between  every  two  sections  ;  these  troughs 
being  of  exactly  the  width  of  a  coil,  are  then  readily 
wound  with  the  remaining  number  of  coils. 

In  winding  the  simple  form  shown  in  figure  17,  in 
which  each  coil  occupies  the  whole  depth,  proceed  as  fol- 
lows :  Suspend  the  armature  core  with  its  shaft  on  two 
high  lathe  centres,  or  on  two  similar  centres  between  two 
strong  posts,  and  jam  it  so  tightly  that  it  requires  some 
force  to  revolve  it.  Select  from  figure  22  the  method 
which  is  to  be  used  ;  divide  the  armature  surface  into  the 
required  number  of  sections  and  mark  on  it,  with  the  same 
number  or  letter,  the  two  opposite  sections  which  are  to 
contain  the  first  coil  ;  similarly  for  all  the  others  as  in  fig- 
ure IV.  Then,  if  there  are  partitions  between  each  of  the 
coils,  as  in  figure  17  or  18,  start  at  the  upper  section  No. 

1,  and   wind  one    coil,  connecting   its    end    temporarily 
with  its  beginning,  and  marking  them,  if  desired,  B  and  E. 
Then   turn  the  armature  around  half  a   revolution,  and 
starting  in  the  same  way  wind  the  neighboring  coil  No. 

2,  figure   1 7,  precisely  similarly,  connecting  its  ends  to- 
gether   and    marking  them    as   before.      Then  turn  the 
armature   again    for   half    a    revolution    and   wind   No. 

3,  and    proceed  thus,    turning  the    armature   one-half  a 
revolution  after  each  coil  is  completed.     When  it  is  com- 
pleted, there  will  be  a  regular  series  of  beginnings  and 
ends  of  coils  along  the  periphery  of  the  armature,  and  they 
will  all  be  in  their  proper  relative  positions.     Untie  the 
ends  and  connect  the  one  marked  E  (end)  of  any  one  coil 
with  the  next  one  marked  B  (which  will  be  in  the  next  but 
one  section)  and  with  one  commutator  bar  ;  the  one  marked 
E  of  this  coil  connect  with  the  next  B,  and  so  on.     By 
winding  it  as  described  it  will  be  found  to  be  almost  im- 
possible to  make  a  mistake  in  the  connections  or  in  the 
proper  location  of  the  coils  belonging  to  different  sides  of 
the  commutator. 

Another  method  is  to  wind  first,  sections  1,  3,  5,  etc., 


96  Principles  of  Dynamo-Electric  Machines. 

figure  17,  without  turning  the  armature  through  half  a 
revolution  each  time  ;  then  winding  the  even  numbers, 
beginning  at  the  point  marked  2.  But  this  is  not  as  good, 
as  the  distribution  of  the  unequal  resistances  of  the  sepa- 
rate unequally  long  coils  is  not  so  well  balanced  as  in  the 
first  method.  When  the  heads  are  small  this  difference 
will  be  less. 

In  winding  the  coils  as  in  figure  19,  two  coils  should  be 
wound  at  the  same  time.  Cut  two  wires  to  the  proper 
length,  call  one  the  light  wire  and  the  other  the  dark. 
Start  with  one  turn  of  the  light  wire  (marked  1),  fasten  it, 
turn  the  armature  through  half  a  revolution  and  start 
the  first  turn  of  the  dark  wire  (marked  2);  turn  the  arma- 
ture again  and  wind  the  turn  No.  3  with  the  light  wire 
which  completes  that  coil  ;  turn  again  and  wind  the  turn 
No.  4  with  the  dark  wire,  which  completes  that  coil. 
If  the  order  given  in  the  second  section  is  to  be  used,  the 
only  difference  is,  that  the  armature  is  not  turned  between 
windings  2  and  3,  which  are  taken  with  the  same  wire  ; 
neither  is  it  turned  in  that  case,  between  winding  4  of  that 
section  and  1  of  the  next.  In  the  first  method  it  must 
be  turned  between  every  two  windings  whether  they  be 
both  in  one  section  or  in  two  neighboring  sections. 

If  there  are  no  lugs,  and  the  appliances  in  figure  23  are 
used,  in  which  case  every  alternate  section  should  be 
wound  first,  it  is  evident  that  if  the  winding  in  figure  18 
is  used,  the  armature  should  not  be  turned  after  complet- 
ing a  coil,  as  all  will  then  be  "light"  coils  until  half  of  their 
number  are  completed  ;  in  the  winding  shown  in  figure  19, 
however,  in  which  each  section  is  a  complete  set  of  two 
coils,  proceed  just  as  if  there  were  "no  alternate  section 
omitted. 

If  the  Siemens  winding,  or  any  of  its  modifications  are 
selected,  it  must  be  remembered  that  there  is  a  slight 
irregularity  just  after  completing  the  winding  of  half  the 
number  of  coils.  If  this  is  not  noticed  it  may  result  in 
wrong  connections  or  in  the  unwinding  of  some  coils,  or  in 


Armatures.  97 

objectionable  cross  connections.  In  the  Froehlich  winding 
it  is  necessary  to  guard  against  winding  into  the  wrong 
sections,  as  they  are  not  diametrically  opposite.  In  all 
cases  errors  are  easily  avoided  by  making  a  correct  draw- 
ing first,  numbering  the  sections  and  transferring  these 
numbers  to  the  armature  core. 

If  by  accident  a  coil  has  been  wound  in  the  wrong  di- 
rection, provided  its  location  is  right,  there  is  no  necessity 
to  unwind  it,  as  it  will  answer  just  as  well  to  connect  it  as 
if  the  beginning  were  the  end,  and  the  end  the  beginning. 
This  applies  to  Gramme  armatures  also. 

To  examine  an  armature  for  wrong  connections  of  the 
beginnings  and  ends  of  coils,  which  should  always  be 
done,  place  the  armature  north  and  south,  put  a  small 
compass  needle  off  to  one  side  of  it,  about  on  a  level  with 
the  axis,  and  test  each  coil  as  it  comes  into  the  vertical 
plane,  with  a  tolerably  strong  current,  by  touching  the 
two  upper  commutator  bars,  or  the  two  upper  connections. 
The  compass  should  always  deflect  in  the  same  direction. 
If  it  changes  its  direction  of  deflection,  reverse  the  con- 
nections of  that  coil.  It  is  not  necessary  to  disconnect 
the  commutator  for  this  test,  if  the  current  is  strong 
enough. 

UNIPOLAR    ARMATURES. 

The  term  unipolar  as  applied  to  machines,  has  not 
reference  to  the  polarity  of  the  magnetic  poles,  as  lines  of 
force  always  have  direction,  and  therefore  two  poles.  It 
has  reference,  we  presume,  to  the  polarity  of  the  armature 
which  in  such  machines  is  always  in  one  and  the  same 
direction,  not  reversed  twice  in  every  revolution,  as  in 
ordinary  machines.  The  advantage  thereby  gained  is  that 
the  currents  will  always  be  in  the  same  direction  in  the 
armature  coils,  and  it  is  therefore  not  necessary  to  have  a 
commutator  to  cut  out  and  reverse  the  connections  of  a 
coil  just  as  it  passes  out  of  one  field  into  the  other,  that  is, 
just  as  the  induced  current  in  it  will  be  reversed,  as  in 


98  Principles  of  Dynamo- Electric  Machines. 

ordinary  machines.  All  sparking  is  therefore  avoided,  as 
the  circuit  is  never  opened  or  altered  in  the  machine  itself. 
All  the  evil  effects  of  self-induction  are  also  avoided,  as  there 
is  no  continued  starting,  stopping,  and  reversing  of  cur- 
rent in  the  armature  coils.  The  current  from  such  ma- 
chines is  precisely  like  a  battery  current,  free  from  the 
pulsations  always  accompanying  ordinary  machine  cur- 
rents. A  telephone  connected  to  a  fine  wire  coil,  which 
forms  the  most  sensitive  detector  of  pulsations  in  currents, 
fails  to  detect  the  slightest  variations. 

Strange  to  say,  the  unipolar  machine  in  the  form  of 
Faraday's  disc,  was,  we  believe,  one  of  the  first  electric  ma- 
chines for  currents  ever  invented  ;  yet  it  has  not  until 
quite  recently  been  made  efficient  to  generate  a  current  of 
more  than  a  very  low  potential. 

The  difficulty  encountered  in  the  old  forms  of  unipolar 
machines  was  that  the  lines  of  force  of  the  field  could  be 
cut  only  once  by  the  inductor  or  armature,  and  that  to  ob- 
tain a  potential  greater  than  a  volt  or  two,  it  was  necessary 
either  to  increase  the  speed,  intensity  of  field  and  size  of 
armature  to  an  impracticable  degree,  or  else  to  couple  a 
large  number  of  machines  in  series.  In  the  ordinary  bi- 
polar machines,  the  inductor  is  in  the  form  of  a  coil,  and 
therefore  cuts  the  same  field  a  great  number  of  times  in 
one  revolution,  generally  over  100  times,  thus  making  the 
potential  over  100  times  that  obtainable  from  a  unipolar 
machine  with  the  same  field  and  inductor  velocity. 

Siemens  improved  the  unipolar  machine,  by  making  the 
armature  a  tube  with  a  cylindrical  pole  inside  and  a  hollow 
pole  outside  having  the  opposite  polarity.  The  lines  of 
force  passing  from  one  to  the  other  are  cut  by  the  revolving 
tube,  thereby  generating  a  current  in  one  direction  in  'the 
tube,  which  was  collected  at  the  two  ends.  In  this  ma- 
chine the  pole  areas  were  quite  large,  but  it  is  evident  that 
this  does  not  increase  the  magnetism  more  than  a  very 
little,  unless  the  magnets  themselves  are  increased  also. 
With  such  a  machine  about  two  volts  were  generated  in  a 


Armatures.  99 

tube  several  feet  long.  The  current,  which  is  limited  only 
by  the  resistance  opposed  to  the  two  volts,  was  naturally 
very  great,  being,  we  believe,  several  hundred  amperes  ; 
the  armature  resistance  is,  of  course,  very  small,  the  greater 
part  being  in  the  collecting  brushes. 

Forbes'  improvement  consisted  in  making  an  economical 
field  of  very  great  intensity  and  rotating  a  disc  between 
the  poles.  Two  sucli  discs  in  two  independent  fields  con- 
stituted a  machine.  The  discs  were  of  iron  to  increase  the 
magnetism.  The  polarity  of  the  two  fields  was  such  that  the 
current  was  from  the  periphery  of  one  disc  to  its  centre,  and 
vio.e  versa  in  the  other  disc,  in  order  to  enable  them  to  be 
connected  in  series  by  the  shaft.  The  current  was  collected 
at  the  periphery  of  the  two  discs  by  numerous  brushes. 
With  two  10-inch  discs  this  machine  gave  at  3,000  revolu- 
tions, about  5.5  volts  on  open  circuit,  and  about  2.5  volts 
when  giving  over  3,000  amperes.  If  the  internal  resistance 
had  been  lower  this  fall  of  potential  would  not  have  been 
so  great. 

The  difficulty  in  unipolar  machines  is  to  connect  several 
discs,  bars,  or  wires  of  one  armature,  in  series  with  each 
other,  that  is,  to  make  the  same  inductor  cut  the  same 
field  a  number  of  times.  In  England,  France,  and  Ger- 
many, numerous  patents  have  been  granted  for  such  ma- 
chines in  which  this  difficulty  was  supposed  to  have  been 
overcome  by  connecting  the  end  of  one  of  the  bars  or  wires 
of  the  armature,  permanently  to  the  beginning  of  the  next 
by  a  wire,  thus  connecting  them  in  series.  This,  however, 
will  evidently  not  generate  any  useful  electromotive  force, 
as  this  connecting  wire  must  in  all  cases  in  which  the  con- 
nection is  permanent,  also  cut  lines  of  force,  and  exactly 
the  same  number  which  the  inductors  cut,  but  unfortu- 
nately for  the  inventors  this  induction  must  necessarily  be 
in  the  opposite  direction,  thus  completely  neutralizing  the 
other  electromotive  force  induced.  It  has  been  suggested 
to  make  this  connecting  wire  pass  through  a  weaker  field, 
thus  allowing  the  difference  of  the  two  opposing 


100  Principles  of  Dynamo- Electric  Machines. 

electromotive  forces  to  appear  at  the  brushes.  This  is  also 
inoperative,  because  wherever  the  same  lines  of  force  make 
a  less  dense  field,  it  must  be  larger  in  area,  that  is,  it  has 
the  same  number  of  lines  of  force.  It  has  been  suggested 
to  shield  these  wires  by  an  iron  tube,  but  as  this  tube  also 
cuts  lines  of  force,  they  must  also  pass  through  its  centre, 
that  is,  through  the  wire. 

A  machine  was  devised  consisting  of  a  Gramme  ring 
without  a  commutator,  the  wire  being  cut  in  one  place  and 
led  to  two  collecting  rings.  One  pole  completely  encircled 
the  outside  of  the  ring,  while  the  other,  a  revolving  pole, 
was  permanently  connected  to  the  iron  of  the  ring  by  arms 
extending  in  between  the  coils.  The  ring  was  then  rotated. 
It  was  supposed  that  the  lines  of  force  would  pass  out  of 
the  core  between  the  wires,  and  thus  would  not  be  cut  in 
the  reverse  direction  by  the  same  wire.  But  as  might  be 
supposed  the  lines  of  force  did  cut  the  wire  in  two  opposite 
directions.  In  a  small  model  constructed  some  years  ago' 
by  the  writer,  not  the  slightest  current  could  be  detected 
even  by  a  sensitive  galvanometer. 

Faraday's  suggestion  was  to  revolve  two  discs  in  the 
same  field,  in  opposite  directions,  and  then  connect  the  two 
peripheries  together  by  sliding  contacts.  The  two  electro- 
motive forces  will  then  be  in  the  opposite  direction  in  the 
two  discs,  but  in  the  same  direction  with  reference  to  two 
collecting  brushes  at  the  centres  of  the  discs.  This  will 
overcome  the  difficulty,  but  for  more  than  two  discs  it  be- 
comes impracticable. 

Within  the  last  few  years  a  number  of  unipolar  ma- 
chines have  been  invented,  in  which  any  number  of 
armature  wires  or  bars,  may  be  connected  in  series  with 
each  other,  that  is,  the  same  field  may  be  used  a  number 
of  times  by  repeated  inductions  in  the  same  circuit.  The 
principle  is  that  the  current  is  led  off  at  the  end  of 
one  inductor  by  a  sliding  contact  and  by  a  wire  which 
is  fixed  with  regard  to  the  field,  and  therefore  does  not  cut 
lines  of  force  ;  this  wire  is  connected  to  the  beginning  of 


Armatures.  101 

the  next  inductor  by  a  sliding  contact,  and  so  on,  connect- 
ing them  all  in  series.  By  this  means  any  desired  electro- 
motive force  may  be  induced,  while  the  current  may  be 
very  great  owing  to  the  large  and  short  inductors  which 
may  be  used. 

ALTERNATING    CURRENT     ARMATURES. 

In  the  general  type  of  alternating  current  machines  the 
armature  consists  of  a  series  of  coils  or  turns  of  wire  in  one 
plane,  generally  placed  in  a  circle  at  the  circumference  of 
a  disc  which  is  rotated.  On  both  sides  of  these  coils  are  a 
series  of  fixed  field  magnets  with  their  opposite  poles 
toward  each  other  thus  making  a  large  number  of  inde- 
pendent fields  through  which  the  coils  of  the  armature 
move  when  the  disc  is  revolved.  The  field  magnets  being 
so  connected  that  the  polarity  of  each  field  is  in  the  reverse 
direction  to  that  preceding,  the  currents  in  the  arma- 
ture will  be  reversed  continually  as  the  coils  pass  through 
the  successive  fields.  The  armature  coils  are  permanently 
connected  with  each  other,  the  two  terminals  of  the  whole 
series  being  connected  to  two  insulated  rings  on  the  shaft, 
on  each  of  which  a  brush  rests  for  collecting  and  leading 
off  the  current.  The  armature  coils  may  be  with  or  with- 
out iron  cores  ;  in  the  former  case,  the  induction  will  be 
much  greater,  but  the  iron  heats  so  much  that  it  is  often 
omitted.  All  iron  cores,  even  if  carefully  laminated,  will 
heat  if  encircled  by  an  alternating  current. 

Such  an  armature  possesses  the  great  advantage  that 
the  coils  may  be  connected  in  multiple  arc,  in  series,  or  in 
any  combination  of  groups,  thus  permitting  the  same 
machine  to  be  readily  altered  to  give  a  low  tension  and 
large  current,  a  high  tension  and  small  current,  or  any  in- 
termediate grades  that  can  be  obtained  by  possible  group- 
ings of  the  coils. 

The  other,  and  perhaps  chief,  advantage  is  that  the  cur- 
rent is  not  commutated,  and  that  therefore  no  commutator 
is  required,  thus  avoiding  all  sparking,  wearing  off  of 


102  Principles  of  Dynamo-Electric  Machines. 

brushes  and  commutators.  No  setting  of  brushes  is  re- 
quired, as  there  is  no  neutral  line  to  which  they  must  be 
set. 

A  curious  feature  of  some  alternating  current  machines, 
presumably  not  in  all  of  them,  is  that  they  require  more  power 
when  running  on  open  circuit  than  when  working  with 
their  normal  load,  and  that  consequently  the  machines  will 
get  very  hot  on  open  circuit.  On  the  other  hand  when 
they  are  short  circuited  they  run  light,  that  is,  require  very 
little  power  to  turn  them.  If  an  armature  coil  is  to  be  cut 
out  it  should  be  short  circuited  and  not  left  open  as  in  con- 
stant current  machines.  The  explanation  of  this  is,  prob- 
ably, that  the  counter  induction  in  the  coil  is  so  great  when 
short  circuited  that  it  is  almost  equal  to  the  initial  induc- 
tion, and  that,  therefore,  only  the  difference  between  the 
two  will  act  to  generate  a  small  current  in  the  coil. 

In  the  large  Gordon  alternating  current  machine  the 
position  of  the  field  magnets  and  the  armature  is  reversed. 
The  field  magnets  are  on  the  circumference  of  a  large  fly- 
wheel, eight  feet  in  diameter,  and  are  made  to  move  past 
the  armature  coils  which  are  fixed  to  the  frame  on  both 
sides  of  the  magnets.  The  great  advantage  of  this  is,  that 
the  armature  coils  may  be  disconnected,  short  circuited, 
or  grouped  in  any  desired  combinations  while  the  machine 
is  running.  The  coils  are  generally  connected  in  two 
groups  giving  two  currents  which  can  be  used  independ- 
ently of  one  another.  Regulation  may  be  effected  either 
by  varying  the  field  current  or  by  short  circuiting  some  of 
the  armature  coils. 

In  general,  alternating  current  machines  must  have  a  sepa- 
rate exciting  machine  for  the  field  magnets,  as  these  require 
a  constant  current ;  this  is,  of  course,  a  great  objection  to 
their  use.  In  some,  however,  the  current  from  a  few  of 
the  coils  in  the  armature,  is  commutated,  so  that  it  be- 
comes a  constant  current  which  may  then  be  used  for  the 
field. 

The    great    objection    to    alternating    currents,    is   the 


Armatures.  i03 

self-induction  of  solenoids  and  particularly  of  magnets.  For 
this  reason  any  shunt  solenoids  or  magnets  used  in  regulators 
or  cut-outs,  as  in  arc  lamps,  must  be  made  very  large  in  order 
to  have  the  same  power  or  pull.  Any  iron  used  in  con- 
nection with  them  must  be  limited  to  a  small  quantity  and 
must  be  well  laminated,  or  it  will  heat  very  much.  A 
core  for  such  a  solenoid  may  be  made  of  a  roll  of  ferrotype 
iron.  The  counter  electromotive  force  in  a  solenoid  for 
alternating  currents  is  not  a  constant  quantity  like  the  re- 
sistance, but  depends  also  on  the  number  of  alternations, 
that  is,  on  the  speed,  and  on  the  current ;  it  will,  therefore, 
vary  under  any  but  the  perfectly  normal  conditions,  and 
for  this  reason,  it  cannot  be  depended  upon  in  automatic 
regulators,  meters,  and  in  most  measuring  instruments, 
except  when  coils  with  very  few  turns,  or  in  general, 
series  coils,  are  used,  as  distinguished  from  line  wire  or 
shunt  coils.  Such  currents  may  be  measured  with  an 
electro-dynamometer,  and  potentials  with  some  voltmeter 
without  coils,  such  as  the  Cardew,  which  depends  on  the 
expansion  of  a  wire  heated  by  the  current. 

Motors  may  be  run  with  alternating  currents,  but  it  is 
not  advisable  nor  economical  to  do  so,  until  their  present 
forms  have  been  greatly  improved. 


CHAPTER   VI. 

Calculation  of  Armatures. 

FROM  the  general  principles  governing  the  designing  and 
construction  of  armatures,  which  were  given  in  the  prev- 
ious chapter,  it  will  be  seen  that  armatures  for  generating 
a  definite  potential  and  current  may  have  widely  varying 
dimensions,  that  is,  a  number  of  armatures  of  entirely  dif- 
ferent proportions  may  generate  the  same  potential  and 
current;  some  one  of  which  will,  however,  be  better  than 
the  others.  It  is  not  possible,  therefore,  to  lay  down  a 
fixed  set  of  rules,  by  means  of  which  the  best  proportions 
of  the  armature  may  be  determined  directly  at  the  outset 
from  the  amount  of  electrical  energy  to  be  generated. 
This  must  be  ascertained  by  comparing  the  general  calcu- 
lated proportions  of  these  different  armatures  with  one  an- 
other, and  selecting  that  one  which  will  be  the  best  under 
the  circumstances.  For  instance,  one  armature  may  have 
twice  as  many  windings  as  another,  and  will,  therefore,  re- 
quire half  the  intensity  of  field,  or  half  the  speed;  or  it 
may  have  half  the  length,  but  twice  the  diameter;  or  it 
may  have  many  other  combinations  of  the  number  of  wind- 
ings, diameter,  length,  speed,  and  intensity  of  field,  and 
yet  give  the  same  potential  and  current.  The  one  which 
will  be  the  best  depends  on  whether  the  machine  is  to  be 
built  cheaply,  or  for  the  greatest  efficiency;  whether  it  may 
be  large  and  massive,  or  small  and  light;  or  whether  the 
speed  may  be  high,  or  must  be  low;  in  general,  it  depends 
on  the  conditions  which  limit  the  construction  of  the  ma- 
chine. It  will,  therefore,  be  different  under  different  cir- 
cumstances, and  must  be  decided  for  each  case. 

It  is  not  necessary,  however,  to  build  these  different 
forms  in  order  to  find  which  is  the  best;  it  may,  in  most 

(104) 


Calculation  of  Armatures.  105 

cases,  be  ascertained  directly  from  the  calculated  dimen- 
sions. The  best  and  shortest  method  for  ascertaining  these 
proportions  is,  therefore,  to  calculate  one  form,  and  then 
to  find  by  calculation  how  a  variation  of  each  one  of  the 
principal  proportions  will  affect  the  ultimate  dimensions 
and  speed;  it  will  then  be  easy,  in  most  cases,  to  select 
from  these  the  one  which  will  be  the  best  under  the  given 
conditions.  In  this  selection  one  should  be  guided  by  the 
calculated  efficiency  of  the  different  forms,  by  the  general 
proportions  of  well-constructed  armatures,  and  by  the  par- 
ticular style  of  frame  and  armature  of  the  machine. 

It  is  difficult  to  say  what  is  the  best  order  in  which  the 
various  parts  should  be  determined.  It  will  make  very 
little  difference,  however,  provided  the  proportions  are 
afterward  varied,  to  see  whether  they  may  be  improved. 
In  general,  the  following  order  may  be  recommended  for 
cylinder  and  Gramme  armatures.  Assume  first  a  certain  in- 
duction in  volts  per  foot,  which  is  thought  to  be  attainable 
in  that  type  of  machine.  Dividing  this  into  the  total  num- 
ber of  volts  to  be  generated  will  give  the  length  of  the 
active  wire  in  one-half.  Assume  70  to  80  ^  of  the  surface  of  the 
armature  to  be  active,  that  is,  embraced  by  the  pole  pieces. 
Dividing  the  active  length  of  wire  by  this  percentage  gives 
the  total  length  of  wire  on  the  cylindrical  surface  of  the 
armature.  From  the  current  which  is  to  flow  through  the 
armature  determine,  approximately,  the  size  of  the  wire, 
remembering  that  the  two  halves  of  the  winding  are  in 
multiple  arc,  and  that,  therefore,  only  half  the  current 
flows  through  each  wire.  From  the  length  of  wire  on  the 
cylindrical  portion  of  the  armature,  and  from  the  diameter 
of  the  covered  wire,  it  is  easy  to  ascertain  the  diameter 
and  length  of  an  armature  which  will  contain  this  amount 
of  wire  on  its  outside  cylindrical  surface.  The  number  of 
layers  being  always  a  whole  number,  which,  from  the  na- 
ture of  the  armature,  is  limited  to  very  few  values,  and  the 
circumference  of  the  armature  being  some  multiple  of  the 
diameter  of  the  insulated  wire,  it  will  generally  be  found 


106  Principles  of  t>ynamo-Electric  Machines. 

that  two  or,  at  most,  three  trial  calculations  will  enable 
one  to  determine  the  diameter  and  length  of  the  armature, 
and,  therefore,  those  of  the  core. 

For  cylindrical  armatures,  a  simple  deduction  will  show 
that  the  following  relation  is  approximately  correct :  the 
length  of  the  wire,  in  inches,  on  the  cylindrical  surface, 
multiplied  by  its  diameter  in  inches  (including  insulation) 
and  divided  by  the  assumed  number  of  layers,  is  equal  to 
the  cylindrical  surface  of  the  armature,  in  square  inches, 
that  is,  its  length  in  inches  multiplied  by  its  circumference 
in  inches.  This  will  enable  one  to  calculate  the  cylindrical 
surface,  from  which  the  diameter  and  length  can  readily 
be  determined,  as  the  circumference  must  evidently  be 
such  a  multiple  of  the  diameter  of  the  insulated  wire  as, 
when  taken  together  with  the  number  of  layers,  gives  an 
even  number  of  lengths,  equally  divided  among  the  com- 
mutator bars  or  coils. 

Another  method,  which  will  frequently  be  found  to  be 
shorter,  is  to  assume  any  fixed  number  of  commutator  bars, 
turns  per  coil,  and  layers;  this  will  determine  the  circum- 
ference or  diameter  of  the  armature.  From  the  number  of 
wires  obtained  thus,  and  from  the  total  length  required, 
the  length  of  the  armature  may  then  be  readily  ascer- 
tained. If  it  is  found  to  be  absurdly  large  or  small,  it  is 
easy  to  find  what  changes  in  the  assumed  numbers  will 
correct  it. 

It  is  assumed  herein  that  there  are  no  iron  or  wooden 
lugs  or  partitions  between  the  coils  on  the  armature; 
if,  however,  such  lugs  are  to  be  used,  the  circumfer- 
ence of  the  armature,  obtained  as  described,  must  be  in- 
creased by  the  sum  of  the  spaces  occupied  by  these  lugs; 
that  is,  by  the  width  in  inches  of  one  lug,  multiplied  by 
their  number. 

After  having  made  these  preliminary  calculations,  it  is 
necessary  to  ascertain  whether  the  resulting  proportions  of 
the  armature  will  comply  with  other  conditions  which  did 
not  enter  into  the  calculations.  For  instance,  from  the 


Calculation  of  Armatures.  10? 

nature  of  the  armature  determine  the  speed  at  which  it  will 
be  safe  to  run  it,  and  from  this,  together  with  the  diame- 
ter, find  the  inductor  velocity.  From  the  induction  in  volts 
per  foot  assumed  at  the  outset,  and  from  this  inductor  ve- 
locity, determine  what  the  intensity  of  the  field  must  be. 
This  may  be  ascertained  as  follows.  Suppose  the  induction 
assumed  was  1.3  volts  per  foot,  and  the  inductor  velocity 
45  feet  per  second,  then  any  one  of  the  wires  will  move 
one  foot  in  TF  of  a  second.  A  foot  of  wire  generates  1.3 
volts,  and,  as  described  in  the  last  chapter,  it  must  cut  the 
field  at  the  rate  of  130,000,000  lines  of  force  per  second,  or 
in  IT  of  a  second  it  must  cut  ^  of  this,  or  about  2,880,000 
lines  of  force.  As  one  foot  of  wire,  in  moving  a  distance 
of  one  foot,  cuts  through  one  square  foot  of  field,  the  num- 
ber of  lines  of  force  just  determined  must  pass  through 
one  square  foot,  independently  of  the  actual  shape  or  size 
of  the  field;  dividing  this  number  by  144  gives  the  number 
of  lines  of  force  per  square  inch,  that  is,  the  intensity  of 
field  required  to  generate  an  induction  of  1.3  volts  per  foot 
at  a  velocity  of  45  feet  per  second.  In  this  case,  the  in- 
tensity of  field  will  be  about  20,000  useful  lines  of  force 
per  square  inch,  which  intensity  can  readily  be  generated 
by  properly  proportioned  and  economical  magnets. 

From  the  diameter  and  circumference  of  the  armature, 
ascertain,  approximately,  what  percentage  of  the  length  of 
one  complete  winding,  or  coil,  will  be  active,  that  is,  will 
lie  on  the  cylindrical  surface  of  the  armature,  taking  care 
to  allow  for  the  extra  length  required  at  the  heads  of  a 
cylinder  armature,  which  for  short  armatures  with  many 
windings  will  be  no  inconsiderable  amount.  Dividing  the 
length  of  wire  on  the  cylindrical  surface  determined  at  the 
outset,  by  this  percentage,  gives  the  total  length.  From 
this,  together  with  the  size  of  the  wire,  find  the  resistance  of 
the  armature,  allowing  also  for  the  heating.  The  square 
of  the  current,  multiplied  by  the  resistance,  gives  the  loss 
of  energy  in  the  armature,  which  should  be  from  2  to  10$ 
of  the  total  energy  generated  by  the  machine,  depending 


108  Principles  of  Dynamo-Electric  Machines. 

on  its  size  and  on  the  desired  efficiency.  If  it  is  found  to 
be  much  greater,  the  armature  will  probably  heat  badly, 
being  poorly  designed  for  efficiency;  if  it  is  less,  it  shows 
that  a  smaller  armature  could  be  used,  if  desired,  decreas- 
ing some  other  parts  proportionally.  A  more  rational 
method  for  determining  the  diameter  of  the  wire  would  be 
to  find  what  its  resistance  should  be  from  the  allowable 
loss  in  the  armature,  and  from  the  current;  from  this  re- 
sistance, together  with  the  total  length  of  wire,  find  its 
cross-section.  But  this  will  frequently  be  found  less  con- 
venient, as  the  length  cannot  always  be  ascertained  with- 
out first  assuming  some  diameter. 

Having  thus  determined  roughly  the  proportions  of  some 
one  form  of  armature,  which  will  generate  the  desired  po- 
tential, and  will  not  heat  too  much  with  the  required  cur- 
rent, ascertain,  by  varying  such  dimension  as  the  nature  of 
the  machine  will  permit,  whether  the  construction  of  the 
armature  may  be  cheapened,  or  whether  the  efficiency  may 
be  increased,  or  both.  As  a  rule,  it  is  not  necessary  to 
make  these  preliminary  calculations  with  any  very  great- 
degree  of  accuracy,  as  there  are  a  number  of  factors — such 
as  the  self-induction,  the  heating,  the  sparking,  etc. — which 
do  not  enter  into  the  calculation,  but  which  will  affect  the 
results.  In  order  to  allow  for  these,  as  well  as  for  other 
small  errors  in  calculation,  it  is  advisable  to  assume  a 
slightly  higher  potential  at  the  outset.  Any  small  errors 
made  may  be  corrected  in  the  winding  of  the  field  mag- 
nets, provided  such  corrections  are  not  too  great  to  be 
within  the  limits  of  the  field. 

In  series  machines  the  difference  of  potential  at  the  ter- 
minals will  be  the  electromotive  force  of  the  armature  less 
the  losses  in  the  field  coils  and  in  the  armature  itself.  The 
electromotive  force  which  is  used  in  calculating  the 
armature,  should  therefore  be  higher  than  the  difference  of 
potential  required  by  the  work  to  be  done,  by  the  amounts 
lost  in  the  machine  itself.  The  loss  in  the  field  magnets 
may  be  from  two  to  five  per  cent,  for  large  well  built 


Calculation  of  Armatures.  109 

machines  and  up  to  10  or  15  per  cent,  in  smaller  cheaper 
ones.  The  same  may  be  allowed  for  the  armature,  though 
it  is  advisable  to  make  this  loss  as  small  as  possible,  as  the 
sparking  generally  decreases  with  the  watts  lost  in  the 
armature. 

For  shunt  wound  machines  the  electromotive  force  will 
be  reduced  only  by  that  lost  in  the  armature,  but  the  cur- 
rent will  have  to  be  increased  by  the  amount  passing 
through  the  field  coils.  This  may  be  taken  from  two  to 
five  per  cent,  for  large  well  built  machines,  and  10  to  15 
per  cent,  in  smaller  cheaper  ones. 

To  illustrate  the  calculation  of  an  armature  by  this 
method,  assume  that  a  cylindrical  armature  is  to  be  de- 
signed for  supplying  the  current  for  150  lamps  at  100 
volts. 

If  the  lamps  require  .65  ampere  each,  the  current  in  the 
external  circuit  will  have  to  be  .65  X  150=  97.5  amperes. 
If  the  machine  is  to  be  shunt  wound  the  current  required 
for  the  field  magnets  may  be  about  3  per  cent,  of  the  total 
current.  The  armature  having  to  supply  both  lamps  and 
field  will  therefore  have  to  generate  97.5  -|-  3  per  cent,  of 
97.5  =  about  100  amperes.  As  the  machine  will  not  be 
large,  the  proportion  of  energy  lost  in  the  armature  may 
be  taken  as  about  5  per  cent.,  which  will  be  about  5  volts. 
As  the  leads  to  the  lamps  will  diminish  the  potential,  the 
machine  must  generate  a  higher  electromotive  force  in 
order  that  the  lamps  may  have  full  100  volts.  For  this 
loss  in  the  leads  5  per  cent,  may  be  allowed,  thus  making 
the  potential  at  the  machine  105  volts,  and  as  5  per  cent, 
was  allowed  for  the  loss  in  the  armature  itself,  the  total 
electromotive  force  to  be  generated  in  the  armature  must 
be  about  105 -f  5  =  110  volts.  The  armature,  therefore, 
must  generate  100  amperes  at  110  volts. 

If  the  armature  is  to  be  well-proportioned  and  has  not 
too  many  windings  or  too  few  commutator  bars,  an  induc- 
tion of  about  1.2  volts  per  foot  may  be  generated  with 
well-proportioned  field  magnets  and  a  velocity  of  the  wire 


110  Principles  of  Dynamo- Electric  Machines. 

of  about  40  feet  per  second.  The  110  volts  will  therefore 
require  110-*-  1.2=  92  feet  of  active  wire,  which  must  at 
all  times  lie  between  one  pole  piece  and  the  armature  core, 
that  is,  must  be  embraced  by  one  pole  piece.  As  the  wire 
which  lies  in  the  neutral  part  of  the  field  is  about  20  to  25 
per  cent,  of  the  whole  amount  of  wire  on  the  cylindrical 
surface,  the  active  part  is  about  80  to  75  per  cent,  of  the 
whole.  Assuming  75  per  cent.,  the  whole  length  of  wire 
on  one  half  of  the  cylindrical  surface  will  be  92-*-. 75 
=  123  feet.  As  described  before,  the  whole  armature 
wire  is,  by  the  nature  of  the  winding,  divided  into  two 
halves,  which  are  in  multiple  arc,  each  half  has  to  generate 
the  full  potential  of  the  machine,  but  carries  only  half  the 
current.  The  123  feet  determined  as  just  described  from 
the  potential  and  the  volts  induced  per  foot,  is  therefore 
the  length  in  one  of  these  halves  of  the  armature  wire  ;  the 
whole  length  on  the  cylindrical  surface  will  have  to  be 
twice  this,  or  246  feet. 

The  cross-section  of  the  wire  should  be  determined  from 
its  allowable  resistance,  in  order  that  the  armature  shall 
not  absorb  more  than  about  5  per  cent,  of  the  whole 
energy,  but  as  this  would  be  found  to  complicate  the  cal- 
culations, it  is  simpler  to  assume  a  certain  cross-section  per 
ampere,  arid  after  completing  the  calculations,  find  whether 
this  will  give  about  the  proper  resistance,  and  if  not,  the 
proper  cross  section  can  then  be  readily  determined.  As- 
suming 520  square  mils  per  ampere,  which,  for  an  arma- 
ture with  few  layers  will  not  cause  it  to  heat  too  much, 
the  cross-section  of  the  wire  will  be  520  X  50  =  26,000 
square  mils,  as  50  amperes  flow  through  each  half  of  the 
armature,  that  is,  through  each  coil  or  wire.  This  cross- 
section  is  very  nearly  that  of  No.  5  B.  &  s.  wire,  the 
diameter  of  which  is  1 82  mils.  The  insulation  will  increase 
the  diameter  about  15  mils,  thus  making  the  diameter  of 
the  covered  wire  197  mils. 

The  length  and  diameter  of  the  armature  might  be 
determined  as  described  from  its  area,  which  can  be 


Calculation  of  Armatures.  Ill 

calculated  from  the  length  and  diameter  of  the  wire  on  its 
cylindrical  surface,  but  it  will  generally  be  found  to  be 
simpler  to  determine  the  diameter  and  length  by  trial,  as 
follows  :  Assume  that  there  are  two  layers  on  the  arma- 
ture, and  56  coils  or  commutator  bars  ;  these  two  assumed 
proportions  may  afterwards  be  varied  to  find  whether  any 
others  would  be  better.  Try  first  three  turns  per  coil  ; 
this  gives  3  X  56=  168  turns,  which,  when  multiplied  by 
two,  gives  the  total  number  of  lengths  or  wires  lying  on 
the  cylindrical  surface  of  the  armature,  as  each  turn  has 
two  active  lengths,  one  on  each  side  of  the  armature.  This 
again  divided  by  two,  as  there  are  two  layers,  gives  168, 
the  number  of  lengths  or  parallel  wires  in  one  layer  ;  multi- 
plying this  by  the  diameter  of  the  covered  wire  in  inches, 
namely,  .197,  gives  33.1  inches  as  the  circumference  of  the 
inside  layer,  which  is  equivalent  to  a  diameter  of  about 
10^  inches. 

As  there  must  be  246  feet  of  wire  on  the  cylindrical  sur- 
face, made  up  of  168  turns  of  two  lengths  each,  there  will 
be  168  X  2  =  336  parallel  lengths,  thus  giving  246-^336 
=  .733  feet,  or  8.8  inches,  as  the  length  of  the  armature 
core  and  pole  piece.  This  may  be  checked  by  calculating 
the  area  of  the  cylindrical  portion  of  the  armature,  first, 
from  its  diameter  and  length  and  then  from  the  diameter 
of  the  covered  wire,  the  number  of  parallel  wires  and  their 
total  length.  Both  should  be  the  same  ;  the  area  in  this 
armature  is  291  square  inches. 

As  this  diameter,  10^  inches,  is  very  large  for  such  a 
small  machine,  and  quite  large  as  compared  to  its  length, 
8.8  inches,  it  is  probable  that  better  proportions  could  be 
found.  Repeating  the  calculations  for  two  turns  per  coil 
and  56  commutator  bars  gives  the  following  proportions  : 
Diameter,  7  inches  ;  length  of  core,  13.2  inches.  These 
proportions  appear  to  be  much  better,  as  the  length  is  very 
nearly  twice  the  diameter. 

To  find  how  these  proportions  will  satisfy  other  condi- 
tions it  is  necessary  to  assume  some  speed.  This  must  be 


112  Principles  of  Dynamo-Electric  Machines. 

based  on  the  mechanical  construction  of  the  frame,  bear- 
ings, and  armature.  If  rigidly  supported  such  an  armature 
may  be  safely  run  at  1,200  revolutions.  With  this  speed 
and  a  mean  circumference  of  23.2  inches  the  inductor 
velocity  will  be  23.2  -*-  12  -j-  1,200  -*•  60=  38.8  feet  per 
second.  This  is  not  as  high  as  would  be  desirable  for  the 
.best  effect  and  economy,  but  with  such  a  small  diameter  a 
higher  velocity  cannot  be  obtained,  as  it  is  not  advisable 
to  increase  the  speed.  This  low  inductor  velocity  indi- 
cates that  a  larger  diameter  might  give  better  results. 

From  this  inductor  velocity  and  from  the  assumed 
induction  in  volts  per  foot,  determine  the  required  inten- 
sity of  the  field,  in  order  to  see  that  it  is  not  too  high. 
One  foot  of  the  wire  moves  38.8  feet  in  one  second,  and 
therefore  passes  through  or  over  an  area  of  38.8  X  1  = 
38.8  square  feet  in  one  second  ;  in  doing  so  it  generates 
1.2  volts,  as  assumed  at  the  outset.  From  this  it  follows 
that  it  must  cut  1 20,000,000  lines  of  force  in  one  second, 
and  that  these  must  pass  through  this  area  of  38.8  square 
feet  ;  reducing  to  inches  and  dividing  gives  120,000,000 
-f-  (38.8  X  144)  =  21,500  lines  of  force  per  square  inch,  as 
the  required  intensity  of  field,  which  will  induce  1.2 
volts  per  foot  at  a  velocity  of  38.8  feet  per  second.  This 
intensity  is  not  too  high  and  can  readily  be  generated  by 
well-proportioned  magnets.  If  it  is  found  from  a  similar 
machine  that  a  greater  intensity  of  field  may  be  obtained 
economically,  the  induction  in  volts  per  foot  may  be  as- 
sumed proportionately  higher.  For  instance,  if  it  is  found 
that  25,000  lines  of  force  per  square  inch  can  be  generated 
economically,  then  the  induction  may  be  assumed  to  be  1.4 
volts  per  foot,  as  this  is  in  the  same  proportion  to  1.2  as 
25,000  is  to  21,500,  or  in  other  words  25,000  -*-  21,500  X' 
1.2  =  1.4.  This  would  decrease  the  length  of  the  arma- 
ture in  the  same  proportion,  that  is,  instead  of  being  13.2 
inches,  it  will  be  only  1.2  X  13.2  -*-  1.4  =  11. 3  inches,  showing 
the  importance  of  having  the  field  as  intense  as  possible. 

To  find  whether  the  resistance  of  the  armature  is  too 


Calculation  of  Armatures.  113 

high  it  is  necessary  to  know  what  the  total  length  of  the 
wire  is,  including  that  around  the  heads.  From  actual 
measurement  it  has  been  found  that  an  armature  whose 
length  is  about  twice  the  diameter  of  core,  the  wire 
on  the  cylindrical  or  active  surface  is  about  40  per  cent  of 
the  whole  length.  The  length  of  wire  on  the  cylindrical 
surface  has  already  been  found  to  be  246  feet ;  as  this  is 
40  per  cent,  of  the  whole,  dividing  it  by  .  40  gives  about 
615  feet  for  the  total  length.  As  the  resistance  of  No.  5 
B.  &  s.  wire  is  .32  ohms,  per  1,000  feet,  615  feet  will  have 
.1968  ohms,  and  as  the  two  halves  of  the  whole  length 
are  in  multiple  arc,  the  resistance  of  the  armature  will  be 
one-quarter  of  this,  or  .049  ohms.  To  see  whether  this 
resistance  will  absorb  more  than  the  5  per  cent,  allowed 
for  it,  find  how  many  watts  are  absorbed  by  the  armature 
and  divide  it  by  the  total  amount  of  energy  generated. 
That  lost  in  the  armature  is  equal  to  the  square  of  the  cur- 
rent multiplied  by  the  resistance,  or  100  X  100  X  .049  = 
490  watts.  The  total  amount  of  energy  generated  is  100 
amperes  X  110  volts  =  11,000  watts.  Dividing  the  first 
by  the  second  gives  about  4.5  percent.,  when  the  armature 
is  cold,  but  it  will  still  be  within  the  5  per  cent,  when  warm. 
These  calculations  show  that  this  armature,  whose  core 
is  about  7  inches  in  diameter  and  13^  inches  long,  will 
answer  all  the  conditions  and  will  not  be  out  of  propor- 
tion. But  in  order  to  be  assured  that  no  other  proportions 
will  give  better  results,  they  should  be  varied,  and  the  cor- 
responding changes  made,  to  see  how  it  will  alter  the  final 
proportions.  For  instance,  fewer  commutator  bars,  as  for 
example,  48,  and  say  three  turns  per  coil,  might  give  better 
proportions  ;  there  will  be  48  X  3  =  144  turns,  as  against 
112  in  the  former  case.  This  would  evidently  make  the, 
diameter  larger  and  shorten  the  length.  A  larger 
diameter  enables  a  greater  inductor  velocity  to  be  obtained 
with  the  same  speed  of  revolution,  thus  increasing  the 
induction  per  foot,  and  thereby  decreasing  the  length  of 
wire.  By  completing  the  calculations  and  making  the 


114  Principles  of  Dynamo-Electric  Machines. 

deductions  as  described,  it  can  readily  be  determined 
which  form  would  probably  be  the  better. 

As  it  is  desirable  to  increase  the  diameter  of  the  arma- 
ture in  order  to  get  a  greater  velocity  with  the  same  num- 
ber of  revolutions,  the  following  proportions  suggest  them- 
selves :  Assume  64  coils  of  two  turns  of  No.  5  B.  &  s. 
wire.  This  gives  128  turns,  and  a  diameter  of  eight 
inches.  With  the  same  strength  of  field  the  induction, 
1.2  volts  per  foot,  will  therefore  be  increased  in  the  pro- 
portion of  7  to  8,  giving  1.4  volts  per  foot.  This,  for  110 
volts,  makes  the  length  of  the  armature  10  inches.  The 
total  length  of  wire  (assuming  that  only  38  per  cent,  is 
active  on  account  of  the  shorter  length)  is  550  feet,  and  the 
resistance  of  the  armature  .044  ohms.  This  is  less  than 
before,  showing  that  with  these  new  proportions  less  wire 
will  be  required,  there  will  be  more  coils,  and  as  the  resist- 
ance is  less  there  will  be  less  energy  lost  in  the  armature, 
and  therefore  less  heating  ;  as  the  heat  radiating  surface 
is  about  the  same  and  the  velocity  greater,  the  heating 
will  be  reduced  still  more.  The  field  is  the  same  strength 
as  before.  If  these  new  proportions  do  not  affect  un- 
favorably the  construction  and  proportions  of  the  frame, 
they  are  evidently  better  than  those  first  determined. 

If  it  is  desired  to  have  only  one  layer,  the  following 
proportions  suggest  themselves  :  56  coils  of  two  turns  and 
but  one  layer  will  evidently  make  the  diameter  twice  as 
large  as  before  with  two  layers.  This  would  probably 
make  it  too  large.  The  number  of  coils  of  two  turns  must 
either  be  taken  less,  say  48,  or  it  must  be  greater  and  have 
but  one  turn  per  coil.  72  coils  is  probably  the  largest  num- 
ber practical  for  such  a  small  machine.  The  diameter  of 
the  wire  may  evidently  be  less  in  this  case,  as  the  heat  is  all 
generated  in  one  layer.  Assuming  72  coils,  one  turn  -per 
coil,  and  a  No.  7  B.  &  s.  wire,  gives  7£  inches  for  the 
diameter  of  the  armature,  and  a  length  of  over  20  inches. 
This  is  undoubtedly  too  long  to  beVun  safely  except  with 
a  very  stiff  shaft. 


Calculation  of  Armatures.  115 

A  No.  6  wire  gives  8.15  inches  for  the  diameter,  and 
about  1 7  inches  for  the  length,  the  induction  being  assumed 
to  be  1.4,  instead  of  1.2,  on  account  of  the  larger  diameter 
or  inductor  velocity.  The  resistance  of  the  armature  will 
be  about. 049  ohms.,  which  is  about  the  same  as  that  of  the 
first  form  calculated,  which  had  56  coils  of  two  turns  and 
two  layers.  The  total  length  of  the  wire  is  about  475 
feet,  as  against  615  feet  of  larger  wire  in  the  other  form, 
showing  a  great  saving  of  wire. 

Numerous  other  proportions  could  also  be  tried  ;  those 
which  are  best  will  depend  on  the  nature  of  the  machine, 
and  must  be  determined  in  each  case  by  calculation  and  by 
the  judgment  of  the  designer. 

Particular  attention  should  be  given  to  making  the  field 
as  strong  as  possible.  In  general,  an  increase  in  the  field 
increases  the  induction  in  the  same  proportion,  and 
decreases  the  necessary  length  of  the  armature  in  about 
the  same  proportion.  This  enables  the  diameter  of  the 
wire  to  be  diminished,  which  in  turn  makes  the  whole 
armature  smaller  and  less  expensive.1 

1  For  the  proportions  of  some  well-built  machines  see  Appendix  I. 


CHAPTER    VII. 

Field  Magnet  Frames. 

THE  general  principles  underlying  the  construction  and 
designing  of  electro-magnets,  have  been  given  in  chapters 
iii  and  iv.  As  the  field  magnets  of  a  dynamo  usually 
serve  also  as  a  frame  work  for  the  machine,  their  design 
and  construction  should  be  based  on  mechanical  considera- 
tions as  well  as  to  meet  the  requirements  of  good  magnets. 
For  instance,  it  is  well  known  that  wrought-iron  magnets 
of  the  same  size  as  those  of  cast-iron  are  more  powerful 
and  more  economical,  but  if  the  nature  of  the  frame  of 
the  machine  which  constitutes  these  magnets  be  such  as 
to  increase  the  cost  very  greatly,  it  is  evident  that  it  will 
be  more  economical  in  such  cases  to  use  cast-iron  and  to 
make  the  magnets  larger.  A  cast-iron  magnet  can  in  all 
cases  be  made  to  generate  as  strong  and  large  a  field  as 
one  of  wrought-iron  (though  not  of  the  same  intensity  per 
square  inch)  by  simply  makkig  it  as  much  larger  as  is  re- 
quired by  its  smaller  capacity.  The  efficiency  of  the  cast- 
iron  magnets,  that  is,  the  amount  of  useful  magnetism 
generated  per  watt  of  electrical  energy  in  the  coil,  may 
not  be  as  great  as  the  efficiency  of  wrought-iron  magnets, 
partly  because  they  are  not  capable  of  carrying  the  same 
number  of  lines  of  force  per  square  inch  of  cross-section, 
and  partly  because,  being  larger  in  diameter,  the  wire  of 
the  coil  has  greater  length  and  resistance.  But  as  4he 
amount  of  energy  consumed  in  the  field  is,  in  well  built 
machines,  only  a  small  percentage,  it  would  not  add  very 
much  to  the  efficiency  of  the  machine  to  reduce  this  already 
small  percentage  by  employing  the  more  efficient  and  more 

(116) 


Field  Magnet  Frames.  117 

expensive  wrought-iron  magnets.  In  this,  as  well  as  in  a 
number  of  other  points,  it  is,  therefore,  a  mere  matter  of 
choice  between  first  cost  as  against  cost  of  running,  and  it 
may  in  most  cases  be  determined  by  the  designer  either 
by  trial  or  by  approximate  calculations. 

The  general  rule  regarding  the  quality  of  the  iron  for 
the  field  magnets  is  to  have  it  as  pure  and  as  soft  as  prac- 
ticable, considering,  as  just  described,  both  the  first  cost 
and  the  gain  in  the  magnetic  qualities.  Wrought-iron 
which  has  been  rolled,  usually  has  a  "  grain "  somewhat 
similar  to  that  of  wood,  the  fibres  running  in  the  direction 
of  the  length  of  the  bar.  Such  iron  magnetizes  better  in 
the  direction  of  the  fibre,  than  across  the  grain,  and  it  is 
therefore  preferable,  wherever  it  is  possible,  to  so  place  the 
iron  that  the  lines  of  force  run  parallel  to  the  grain. 
Wrought-iron  in  the  form  of  fine  wire  or  thin  sheets  may 
usually  be  relied  upon  as  being  soft,  but  in  this  form  its 
use  is  limited  to  small  machines  and  to  armature  cores. 

When  cast-iron  is  used  it  should  be  as  soft  and  as  free 
from  impurities  as  possible.  It  is  preferable  whenever 
possible,  to  have  it  annealed,  and  when  not  too  large  in 
bulk,  to  have  it  converted  into  malleable  iron  ;  this  is  espe- 
cially to  be  recommended  for  small  machines  and  motors. 
Corners,  projections,  and  thin 'edges  should  be  avoided  as 
much  as  possible  on  the  castings  as  they  are  apt  to  chill 
while  being  cast,  thus  making  them  quite  hard,  and  destroy- 
ing their  magnetic  qualities.  Corners  and  edges  should 
be  well  rounded  off,  and  whenever  thin  projecting  edges 
are  necessary  for  mechanical  reasons  they  should  be  cast 
quite  thick  and  massive,  and  may  afterwards  be  planed  or 
turned  down  if  necessary.  It  is  preferable  to  cast  the  iron 
in  dry  moulds  and  to  allow  it  to  cool  as  slowly  as  possible, 
preferably  for  three  or  four  days  in  a  gradually  decreasing 
fire. 

The  cores  of  the  magnets  are  usually  the  parts  in  which 
the  lines  of  force  are  most  dense,  as  it  is  the  smallest  cross- 
section  through  which  the  magnetism  has  to  pass.  A  very 


118  Principles  of  Dynamo-Electric  Machines. 

good  and  economical  magnet  can  therefore  be  constructed 
by  making  these  cores  of  wrought-iron  and  the  pole  pieces 
and  yoke  pieces  of  cast-iron. 

The  relative  value  of  cast  and  wrought-iron  magnets 
may  be  deduced  from  the  following  figures.  Sylvanus 
Thompson  states  that  cast-iron  magnets  will  give  about  60 
per  cent,  of  the  effect  of  wrought-iron  magnets  of  equal 
size.  For  instance,  if  a  wrought-iron  magnet  can  have 
100,000  lines  of  force  passing  through  every  square  inch  of 
its  cross-section  of  core,  at  saturation,  a  cast-iron  one  will 
have  60,000  at  saturation  ;  to  be  equal  to  the  wrought-iron 
magnet  it  would  have  to  be  two-thirds  again  as  large  in 
cross-section  of  core,  because  every  If  square  inches  will 
then  contain  the  same  number  of  lines  of  force  as  one 
square  inch  of  the  wrought-iron  core.  A  wrought-iron 
magnet,  according  to  this  statement,  need  be  only  60  per 
cent,  or  three-fifths  as  large  in  cross-section  as  a  cast-iron 
one  equal  to  it  in  effect.  The  length  of  the  core  will  de- 
pend in  general  only  on  the  amount  of  wire  which  it  is 
necessary  to  wind  around  it  in  order  to  generate  this 
magnetic  effect  in  the  core. 

Kapp1  states  that  in  two  similar  machines,  differing  only 
in  having  the  field  magnets  of  the  one  made  of  cast-iron 
and  of  the  other  of  wrought-iron,  the  electromotive  forces 
generated  were  80  and  100  volts  respectively.  As  all 
other  conditions  were  the  same,  it  follows  that  the  result 
must  be  due  solely  to  the  magnetism,  and  that,  therefore, 
the  cast-iron  has  80  per  cent,  of  the  effect  of  wrought-iron. 
According  to  this,  a  cast-iron  core  should  be  one-quarter 
or  25  per  cent,  larger  in  cross-section  than  an  equivalent 
wrought-iron  one,  and  vice  versa,  a  wrought-iron  one 
should  be  80  per  cent,  or  four-fifths  as  large  as  a  cast-iron 
one. 

To  determine  the  actual  size  of  electro-magnets,  one 
should  be  guided  by  the  following  important  property  of 

1.  Electrician,  London,  May  22,  1885,  p.  23. 


Field  Magnet  Frames.  119 

such  magnets.  In  passing  a  current  through  the  coil  of  an 
electro-magnet,  and  varying  its  strength  from  0  amperes 
to  the  greatest  possible  current,  it  will  be  found,  on  meas- 
uring the  magnetism  corresponding  to  each  current,  that 
at  first  it  increases  quite  rapidly,  and  very  nearly  in  pro- 
portion to  the  current,  that  is  for  double  the  current  there 
will  be  about  double  the  magnetism.  This  will  be  the 
case  up  to  a  certain  point,  when  the  conditions  will  sud- 
denly change,  and  on  increasing  the  currents  still  more  the 
magnetism  will  increase  only  very  slightly.  At  this 
point,  called  the  point  of  saturation,  the  action  of  the  iron 
in  adding  its  share  to  the  magnetism,  appears  to  cease  ;  all 
the  increase  above  this  point  appears  to  be  that  due  to  the 
current  itself,  as  if  the  magnet  were  a  mere  solenoid,  or 
coil  without  iron.  Up  to  this  point  the  magnetism  is  ob- 
tained cheaply  and  economically,  but  above  this  point  the 
increase  in  magnetism  is  so  small  and  requires  such  a  large 
expenditure  of  current,  electromotive  force  and  wire,  that 
it  is  prohibitive  in  well  built  and  efficient  machines.  It  is 
of  very  great  importance,  therefore,  to  guard  against  over- 
saturating  to  any  great  extent  the  magnets  or  any  part  of 
the  magnetic  circuit  of  the  field  magnet  frames,  as  it  will 
always  be  found  that  it  takes  less  copper  or  less  electrical 
energy,  to  generate  the  same  amount  of  magnetism  in  a 
larger  core  which  is  not  over-saturated  than  in  a  small 
over-saturated  core.  This  is  especially  important  in  small 
or  cheap  machines,  or  in  those  in  which  there  is  a  scant 
allowance  of  iron  in  the  magnetic  circuit.  As  the  actual 
additional  weight  of  iron  required  to  bring  the  magnetiza- 
tion of  an  over-saturated  magnet  down  to  the  saturation 
point,  is  so  small,  and  therefore  inexpensive,  there  is  in 
general  no  reason  why  it  should  not  be  added.  As  iron 
itself  appears  to  add  to  the  lines  of  force  which  are  due  to 
the  current  alone,  there  is  an  additional  advantage  in  using 
as  much  iron  as  possible,  the  magnetism  of  the  iron  itself 
requiring  very  little  electrical  energy  to  render  it  useful. 
When  cast-iron  magnets  are  used  there  is  a  still  further 


120  Principles  of  Dynamo- Electric  Machines. 

advantage  in  using  larger,  massive  magnets,  as  the  iron  is 
more  apt  to  be  cast  soft  than  when  they  are  small  and 
thin.  How  to  find  out  whether  the  magnets  are  over- 
saturated  too  much  will  be  explained  in  a  subsequent 
chapter.  Some  makers  prefer  to  over-saturate  the  magnets 
to  a  slight  extent  because  the  field  is  then  less  sensitive  to 
slight  changes  of  the  current  in  the  coils  ;  in  other  words, 
the  machine  is  steadier  in  its  action.  If  this  is  to  be  done 
it  should  be  to  a  slight  extent  only.  But  as  machines 
are  generally  regulated  by  altering  the  current  in  the 
field,  it  would  seem  that  it  is  desirable  to  have  the  field 
as  sensitive  to  variations  of  current  as  possible,  in  order 
that  it  shall  require  less  variation  of  this  adjusting  or 
regulating  current. 

Scientists  have  up  to  the  present  time  failed  to  give 
the  dynamo  builder  any  practical  and  reliable  data  and 
rules  by  means  of  which  the  actual  sizes  of  the  parts  of 
a  field  magnet  frame  may  be  determined  with  any  degree 
of  certainty  from  the  intensity  and  size  of  the  field  re- 
quired by  the  armature.  In  the  absence  of  this  greatly 
needed  information,  the  designer  of  dynamos  will  have  to 
use  his  judgment  if  he  has  no  access  to  a  finished  model 
machine  from  which  to  obtain  the  desired  data.  The  fol- 
lowing relations  may  be  used  as  a  guide  in  determining 
the  approximate  sizes. 

Every  line  of  force  must  make  a  complete  closed  circuit 
around  its  exciting  current,  as  explained  in  chapter  iii. 
For  instance,  in  the  well-known  form  of  magnet  frame 
shown  in  figure  24,  the  lines  of  force  will  pass  through  the 
iron  as  shown  in  dotted  lines,  the  only  portion  of  them 
which  is  rendered  useful,  that  is,  which  may  be  cut  by  the 
armature  wire,  is  that  passing  from  the  pole  pieces  through 
the  air  space  to  the  armature  and  out  at  the  other  pole- 
piece.  Iron  is  saturated  when  a  certain  number  of  lines 
of  force  pass  through  every  square  inch  of  its  cross-section ; 
if,  therefore,  the  iron  was  of  the  same  quality  throughout, 
and  if  these  useful  lines  of  force  were  the  only  ones,  it  is 


Field  Magnet  Frames. 


121 


readily  seen  that  the  cross-section  of  the  yoke  piece  at  I  b, 
should  be  at  least  equal  to  that  of  the  core  at  a  a  ;  the 
cross-section  of  the  pole-piece  at  e  e,  should  be  at  least  twice 
that  at  a  a,  as  it  contains  the  lines  of  force  from  both  halves 
of  the  frame,  as  seen  from  the  dotted  lines.  If  a  Gramme 
ring  armature  is  used,  the  lines  of  force  divide  equally,  half 
of  them  going  through  each  half  of  the  ring  ;  its  cross- 
section  c  c  =  d  dy  ought,  therefore,  to  be  equal  to  a  a  ;  if  it 


"Fig.    24 


is  a  cylinder  armature  the  whole  cross-section  c  cdd should 
be  equal  to  twice  a  a.  As  the  lines  of  force  generated  in 
the  magnet  a  a,  return  through  the  other  core  below  it,  it 
follows  that  in  such  a  frame  each  core  must  be  made  large 
enough  to  contain  all  the  lines  of  force  generated  in  it  and 
in  the  other  core  belonging  to  that  pair,  considering  them 
as  the  two  parts  of  a  jj  magnet.  In  other  forms  of 
frames,  it  may  be  readily  determined  what  the  relative 
cross  sections  should  be,  by  merely  following  the  lines  of 


122  Principles  of  Dynamo-Electric  Machines. 

force  throughout  their  circuit  and  giving  each  line  of  force 
the  same  area  of  iron  to  pass  through.  For  instance,  in 
the  Edison  form,  one  pair  of  cores  take  the  place  of  the 
two  pair  in  figure  24,  and  should  therefore  be  twice  as  large 
in  cross-section. 

Referring  to  figure  24,  it  will  be  noticed  that  a  few  of 
the  lines  of  force,  as  m,  ra,  n,  n,  do  not  pass  through  the 
armature  and  are  therefore  lost,  representing  so  much  leak- 
age, as  it  might  be  called.  But  as  these  all  pass  through 
the  cores  of  the  magnets,  allowance  should  be  made  for 
them  in  the  cross-section  of  these  cores  and  they  should 
therefore  be  larger  in  comparison  to  the  core  of  the  arma- 
ture, than  if  there  was  no  leakage,  as  was  supposed  in  the 
above  mentioned  rule.  It  is  not  possible  to  state  just  how 
much  to  allow  for  this  leakage,  as  it  depends  greatly  on 
the  detailed  parts.  For  rough  calculations  it  might  be 
estimated  that  in  the  forms  shown  in  figure  24,  about  one- 
quarter  of  the  total  number  of  lines  of  force  are  lost  by 
this  leakage,  in  which  case  the  cross-section  at  a  a,  should 
be  one-third  again  as  large  as  that  at  c  c. 

The  iron  of  the  armature  is  generally  of  much  finer 
quality  than  tha£  of  the  field  magnets,  and  therefore  its 
cross-section  per  line  of  force  may  be  made  smaller.  Tak- 
ing the  results  of  Kapp's  experiments  for  wrought  and  cast- 
iron,  given  above,  it  would  follow  that  the  cross-section  of 
a  wrought-iron  armature  may  be  made  80$  of  that  of  a 
corresponding  cross-section  of  a  cast-iron  magnet  core. 
This  would  still  further  increase  the  cross-section  at  a  a, 
figure  24,  over  that  at  c  c.  On  the  other  hand  the  cross- 
section  of  the  armature  core,  which  ought  to  be  laminated, 
is  not  entirely  composed  of  iron  ;  allowance  should  there- 
fore be  made  for  the  spaces  occupied  by  the  insulating 
layers,  or  the  air  ventilation  space. 

There  is  no  objection  to  making  the  cross  sections  at  any 
point  larger  than  that  which  would  be  required  by  the  rules 
just  given,  provided  it  does  not  uselessly  increase  the  length 
of  wire  on  the  coils  or  armature.  The  important  point  is 


Field  Magnet  Frames.  123 

that  they  should  not  be  smaller  than  that  required  by  these 
rules.  For  instance,  the  yoke  piece  at  b  b,  or  the  pole- 
piece  at  e  e  may  be  made  as  much  greater  as  is  desired, -the 
effect  will  even  be  advantageous,  as  it  adds  to  the  number 
of  lines  of  force  inherent  in  the  iron.  The  two  places 
where  there  should  be  no  more  iron  than  is  absolutely 
necessary  are  the  cross  sections  of  the  cores  a  a,  and  that 
of  the  armature  c  c.  The  latter,  however,  may  be  limited 
by  the  nature  of  the  armature  and  its  winding,  and  for 
cylinder  armatures  it  may  often  have  to  be  greater  than 
that  required  by  the  number  of  lines  of  force  ;  in  Gramme 
armatures,  on  the  other  hand,  it  is  often  much  too  small  for 
the  passage  of  the  required  number  of  lines  of  force  and  is 
therefore  saturated  long  before  the  field  magnets  are  at 
their  maximum  intensity  ;  this  causes  neutralizing  reverse 
currents  in  the  inside  wires,  great  leakage  of  otherwise 
useful  lines  of  force,  and  consequent  bad  sparking  in  the 
otherwise  dead  or  short  circuited  coils  in  the  neutral  field. 
The  objection  to  increasing  the  cross-section  of  the  cores 
at  a  a,  above  what  is  absolutely  necessary  is,  that  it  in- 
creases uselessly  the  length  of  the  wire  of  the  coils,  which 
in  well  built  machines  is  an  important  item  in  the  cost  of 
the  material  for  a  machine. 

The  armature  current  itself  generates  quite  a  number  of 
lines  of  force  in  its  core.  These  partly  leak  through  the 
air  and  return  to  the  other  side  of  the  core,  but  most  of 
them  return  through  the  frame  of  the  field  magnets 
together  with  the  other  lines  of  force,  and  in  the  same 
direction.  They  therefore  help  to  saturate  the  iron.  If 
this  counter-magnetization  of  the  armature  is  small,  as  it 
should  be  in  well  built  machines,  the  effect  will  not  be 
great  ;  but  in  armatures  with  many  turns  of  wire  the  effect 
is  not  small  and  should  be  allowed  for  by  an  additional 
increase  in  the  cross-section  of  the  iron. 

In  the  foregoing  chapters  it  was  shown  how  the  number 
of  lines  of  force  required  by  the  armature  may  be  calcu- 
lated approximately.  If  it  were  known  how  many  lines 


124  Principles  of  Dynamo-Electric  Machines. 

of  force  could  be  generated  per  square  inch  of  cross-sec- 
tion of  magnet  core  for  different  qualities  of  iron,  and  if  it 
were  known  how  many  of  the  lines  of  force  were  wasted 
by  leakage,  it  would  be  very  easy  to  calculate  the  neces- 
sary cross-section  of  the  field  cores.  No  complete  and  re- 
liable set  of  such  figures  have  been  published,  but  the 
following  may  serve  as  a  rough  guide.  Kapp2  states  that 
in  wrought-iron  field  magnets  of  hammered  scrap,  108,000 
lines  of  force  may  be  passed  per  square  inch  of  cross-sec- 
tion ;  in  armatures  of  charcoal-iron,  well  annealed,  150,- 
000,  and  of  discs  of  similiar  iron,  132,000.  Fleming  states 
that  in  a  long  magnet  of  soft  annealed  iron,  the  greatest 
strength  is  116,000  lines  of  force  per  square  inch.  The 
same  authority  states  that  in  the  best  dynamo  the  intens- 
ity of  magnetization  is  from  about  40,000  to  65,000  lines 
of  force  per  square  inch,  by  which  we  suppose  he  refers  to 
the  magnet  cores  and  not  to  the  armature  field.  Jn  the 
Edison  and  Weston  machines,  tested  by  the  Franklin  In- 
stitute, a  rough  estimate  gives  about  70,000  to  90,000  in 
the  cores. 

If  a  suitable  machine  of  a  similar  style  to  the  one  to  be 
constructed,  is  at  the  disposal  of  the  designer,  it  is  very 
easy  to  determine  from  it,  as  follows,  what  the  cross-sec- 
tion of  the  cores  should  be  for  a  larger  or  smaller  machine 
of  the  same  general  type,  or  of  one  differing  only  in  the 
armature.  Run  the  machine  with  a  separate  exciter  and 
without  a  current  in  the  armature  ;  measure  the  potential 
on  open  circuit  for  gradually  increasing  exciting  currents. 
When  the  increase  in  potential  begins  to  be  much  slower 
than  the  increase  in  the  exciting  current  the  machine  is 
saturated.  With  this  intensity  of  field  at  saturation,  let 
the  armature  current  flow  through  its  proper  external  cir- 
cuit and  measure  its  current  and  potential.  From  this 
and  the  detail  dimensions  of  the  armature  the  number  of 
lines  of  force  in  the  field  can  be  calculated  as  described 

2.  Proceedings  of  Society  of  Telegraph  Engineers,  Nov.  11,  1886,—"  Char- 
acteristics of  Dynamos.1" 


Field  Magnet  Frames.  125 

in  the  previous  chapters.  Knowing  the  intensity  of  field 
required  by  a  new  armature  for  another  machine  it  is  only 
necessary  to  increase  or  diminish  the  cross-section  of  the 
magnet  cores  in  the  same  proportion  as  the  new  field  is 
larger  or  smaller  than  the  one  tested. 

The  length  of  a  magnet  core  depends  only  on  the 
amount  of  wire  which  is  required  in  its  coil  to  generate 
the  necessary  number  of  lines  of  force.  It  is  evident  that 
it  takes  a  curtain  number  of  ampere  turns  to  develop  the 
required  magnetism,  and  owing  to  the  limited  resistance 
of  these  coils  and  to  their  allowable  degree  of  heating, 
the  amount  of  wire  is  also  limited.  If  the  coils  are  very 
short,  the  thickness  of  the  coil  will  be  so  great  that  the 
outside  layers  may  not  have  their  full  effect,  being  too  far 
from  the  iron  ;  it  will  also  make  the  outside  layers  un- 
necessarily long.  On  the  other  hand,  if  the  magnet  core 
is  very  long,  the  thickness  of  the  coil  will  be  very  small, 
which  by  itself  is  an  advantage,  but  the  weight  of  iron 
will  then  be  uselessly  great,  making  the  machine  larger 
and  heavier  than  is  necessary,  while  the  greater  magnetic 
resistance  and  the  greater  length  of  the  course  of  the 
lines  of  force  will  both  tend  to  weaken  the  magnetic 
strength  and  to  increase  the  leakage. 

One  of  the  older  authorities  states  that  the  best  rela- 
tion between  the  coil  and  core  of  a  circular  magnet  is 
when  the  core  is  one-third  of  the  whole  external  diameter 
of  the  magnet,  that  is,  when  the  thickness  or  depth  of  the 
coil  is  equal  to  the  diameter  of  the  core.  But  although 
this  may  be  the  case  for  telegraph  magnets,  it  is  certainly 
not  the  case  with  dynamos,  in  which  the  cost  of  the  wire 
and  the  resistance  of  it  are  of  much  greater  importance 
than  in  telegraph  magnets.  -  A  very  good  rule,  and  one 
which  is  based  on  actual  experience  with  machines,  as 
distinguished  from  theoretical  deductions,  is  to  make  the 
thickness  or  the  depth  of  the  coil  about  one-third  of  the 
diameter  of  the  core  on  round  magnets,  and  one-third  of 
the  smaller  diameter  on  oval  magnets.  //If  made  much 


126  Principles  of  Dynamo-Electric  Machines. 

greater  than  this  the  magnets  will  be  apt  to  heat  too 
much,  or  to  over-saturate  the  core,  or  to  have  a  uselessly 
large  amount  of  wire  on  them.  If  much  less  than  this,  it 
will  generally  be  found  that  the  magnets  are  larger  than 
would  be  necessary  to  answer  the  purpose.  The  latter 
is  no  serious  fault  and  will  not,  as  in  the  other  case,  pre- 
vent the  magnets  from  generating  at  least  the  desired 
amount  of  magnetism,  and  on  the  contrary  will  allow  for 
an  increase  of  magnetism  if  it  should  be  found  necessary  ; 
it  is,  therefore,  much  safer  to  make  the  cores  thicker  and 
longer,  and  consequently  the  depth  of  the  coil  less,  than 
what  might  at  first  appear  to  be  necessary.  Furthermore, 
in  many  forms  of  frames,  the  cores  can  readily  be  short- 
ened if  it  should  be  found  necessary,  while  they  cannot 
always,  so  conveniently,  be  lengthened. 

If  the  number  of  ampere-turns  required  for  a  magnet 
were  known,  it  would  not  be  difficult  to  determine  the 
length  of  the  core,  because,  its  cross-section  can  be  de- 
termined as  described,  from  which  together  with  the  am- 
pere-turns and  the  allowable  thickness  of  coil,  the  length 
is  readily  calculated.  But  in  most  cases  the  ampere- 
turns  are  the  last  thing  to  be  determined,  when  the  ma- 
chine is  otherwise  completed,  and  it,  therefore,  remains 
only  for  the  designer  to  use  his  judgment  regarding  the 
length  which  the  cores  should  have.  Besides  the  remarks 
already  made  concerning  the  length  of  cores,  the  follow- 
ing may  be  used  as  general  guides.  Deprez  states  that  to 
have  the  best  effect  the  length  of  a  magnet  should  not  be 
greater  than  three  times  its  thickness  ;  this  we  presume 
refers  to  the  core  and  not  to  the  outside  dimensions  of 
the  whole  magnet  with  its  coil.  A  short  thick  magnet  will 
magnetize  and  demagnetize  more  readily,  or  as  Fleming 
states  it,  "  such  a  magnet  has  no  magnetic  memory  ;"  it 
responds  more  quickly  to  changes  of  current  in  its  coils. 
Some  makers  consider  this  a  disadvantage  and  purposely 
over-saturate  their  magnets  in  order  to  "  steady  "  them  in 
their  action.  The  tendency  of  reliable  and  systematic 


Field  Magnet  Frames.  127 

dynamo  builders  has  been  toward  short  thick  magnets 
and  it  may,  therefore,  be  accepted  as  a  safe  example  to 
follow. 

The  following  general  principles  governing  the  mag- 
netism of  coils  may  also  serve  as  a  general  guide  in  de- 
signing magnets.  In  a  circular  coil  of  wire  composed  of 
a  turn  or  turns  of  wire,  as  for  instance  in  a  large  tangent 
galvanometer,  if  the  radius  in  centimeters  is  r  and  the 
ampere-turns  are  represented  by  c  then  the  intensity  of 
magnetism  at  the  centre,  in  lines  of  force  per  square  cen- 
timeter, will  be 

2  Ttc 
^ 

which  may  be  reduced  to 

or  if  d  is  the  diameter, 

£=1.25664-r 
a 

Or  if  R  and  D  are  the  radius  and  diameter  in  inches,  and 
/  the  intensity  in  lines  of  force  per  square  inch,  these 
formulae  become 

1=  1.5959-^-  =  3  1918-^- 

In  these  formulae  c  will  be  the  current  in  amperes  if  there 
is  only  one  turn  in  the  coil,  but  if  there  are  many  turns  c 
is  the  current  multiplied  by  the  number  of  turns,  that  is, 
the  ampere-turns.  From  these  formulae  either  the  intens- 
ity, diameter  or  ampere-turns  may  be  calculated. 

The  intensity  in  different  parts  of  the  area  enclosed  by  a 
circular  coil  is  not  the  same,  being  least  at  the  center  and 
greatest  nearest  the  circumference.  If  the  ratio  which  the 
mean  or  average  intensity  bears  to  that  at  the  center  is  known, 
the  total  number  of  lines  of  force  is  readily  calculated,  it 


128  Principles  of  Dynamo-Electric  Machines. 

being  the  average  intensity  multiplied  by  the  area.  On  the 
other  hand,  if  the  total  number  of  lines  of  force  enclosed  in 
this  circle  is  known,  the  mean  or  average  intensity  is  the  total 
number  of  lines  of  force  divided  by  the  area. 

Expressing  this  in  formulae,  if  h  is  the  mean  intensity  per 
square  centimeter,  or  H  that  per  square  inch,  and  if  k  is  the 
ratio  of  this  mean  intensity  to  that  at  the  center,  then 

h=ik 

and  H=Ik 

If  M  is  the  total  number  of  lines  of  force  in  the  whole  circle, 
then 

M=ik  TT  r*=1.9739  &cr=.98695M ; 
or,  if  the  dimensions  are  in  inches, 

^=5.0137  fo.K=2.5069  kcD. 

These  formulae  are  correct  only  for  a  coil  in  which  the  cross- 
section  of  the  coil  space  is  small.  For  long  or  thick  coils 
they  are  not  strictly  correct,  but  may  serve  as  a  general  guide 
in  calculating  such  coils,  and  can  in  general  be  relied  upon 
for  relative  proportions  between  two  coils  or  magnets.  For  a 
detailed  discussion  of  this  subject  the  reader  is  referred  to  the 
more  advanced  scientific  treatises. 

Numerous  deductions  can  be  made  from  these  formulae, 
some  of  which  may  serve  as  guides  in  the  construction  of 
magnets.  For  instance,  if  the  current  and  the  diameter  of 
the  coil  remain  the  same,  the  resistance  of  the  coil  does  not 
affect  the  magnetism  which  is  generated.  By  increasing  the 
size  of  the  wire  of  a  coil  the  energy  in  watts  may  be  reduced 
to  any  amount,  while  the  magnetism  remains  the  same ;  or  if 
the  current,  and  therefore  the  magnetism,  is  increased  when 
larger  wire  is  used,  so  that  the  number  of  watts  consumed  re- 
mains the  same,  then  it  is  evident  that  the  amount  of  magnet- 
ism obtainable  per  watt  of  energy  may  be  greatly  increased, 
showing  that  there  is  no  fixed  relation  between  the  amount 
of  magnetism  generated  and  the  watts  required  to  generate 
it,  and  that  theoretically  any  amount  of  magnetism  may  be 
obtained  per  watt,  it  being  limited  in  practice  only  by  the 
allowable  size  of  the  coil  space  and  the  cost  of  the  copper. 


Field  Magnet  Frames..  129 

When  iron  is  introduced  into  the  coil,  as  in  an  ordinary 
electro-magnet,  the  intensity  and  total  number  of  lines  of 
force  will  be  increased  greatly.  The  iron  acts  as  if  it  con- 
tained a  large  amount  of  magnetism  in  it,  which  is  ren- 
dered active  by  passing  a  current  of  electricity  near  it.  Its 
action  may  be  pictured  as  follows  :  suppose  a  quantity  of 
steel  filings  were  magnetized  very  strongly  and  placed  in  a 
round  bottle  containing  thick  syrup  or  any  other  material 
which  will  suspend  them  but  which  will  not  allow  them  to 
turn  around  unless  some  force  is  exerted  on  the  particles 
of  steel.  Each  piece  representing  an  enlarged  molecule  of 
iron,  being  a  magnet,  will  attach  itself  to  others  of  oppo- 
site polarity,  thus  forming  complete  little  magnetic  cir- 
cuits amongst  themselves.  If  well  shaken  they  will  attach 
themselves  as  described  and  will  therefore  exhibit  no  defi- 
nite polarity  to  a  needle  placed  outside,  though  they  will 
attract  the  needle.  This  represents  iron  in  its  normal  con- 
dition. If  a  coil  be  wound  around  this  bottle  and  a  current 
be  passed  through  it,  these  little  magnets  will  turn  on  their 
axis  and  arrange  themselves  perpendicular  to  the  wire  of 
the  coil  and  parallel  to  the  lines  of  force  generated  by  the 
current.  They  will  therefore,  each  add  their  magnetic 
intensity  to  that  of  the  current,  as  they  have  arranged 
themselves  in  the  same  direction.  They  will  then  exhibit 
polarity  to  a  needle  outside  of  the  bottle,  the  polarity  being 
the  same  as  that  of  the  coil.  The  stronger  the  current  the 
greater  will  be  the  force  to  turn  their  particles  out  of  their 
normal  position  and  into  that  induced  by  the  coil.  Their 
external  magnetism  will  therefore  increase  with  the  cur- 
rent until  they  have  all  been  turned  parallel  to  each  other, 
when  the  increase  of  magnetism  of  the  particles  will  stop. 
This  corresponds  to  the  point  of  saturation  above  which  the 
magnetism  of  the  iron  itself  cannot  be  increased.  When 
the  current  is  stopped  they  will  again  attach  themselves  to 
each  other  forming  closed  magnetic  circuits,  and  not 
showing  any  external  magnetism. 

The   application   of   the   formulae   just   given   may    be 


130  Principles  of  Dynamo- Electric  Machines. 

illustrated  as  follows  :  Suppose  the  magnets  of  a  dynamo 
which  has  been  tested,  were  found  to  be  over-saturated 
too  much,  and  it  is  desired  to  construct  a  new  frame  for 
this  machine  in  which  they  were  not  to  be  over-saturated. 
By  the  tests  which  will  be  described  in  a  subsequent  chap- 
ter, find  what  the  magnetism  must  be  for  the  proper  load 
of  the  machine  when  the  magnets  are  over-saturated  ;  call 
this  m.  It  is  not  necessary  to  know  the  actual  number  of 
lines  of  force  but  merely  a  number  which  is  proportional 
to  it,  such  for  instance,  as  the  potential  in  volts  measured 
on  open  circuit,  with  the  magnets  excited  to  the  required  de- 
gree of  magnetization.  Make  another  test  to  determine  the 
magnetism  in  the  magnets  (or  the  potential  on  open  cir- 
cuit) when  they  are  just  saturated  ;  call  this  n,  and  let  the 
number  of  ampere-turns  required  in  this  case  be  c.  From 
n  and  the  area  of  cross-section  of  the  core,  find,  by  dividing 
the  former  by  the  latter,  what  the  intensity  of  magnetiza- 
tion per  square  inch  is,  just  at  saturation.  If  the  number  of 
volts  has  been  used  instead  of  the  number  of  lines  of  force, 
the  number  thus  obtained  for  the  intensity,  will  by  itself 
mean  nothing,  but  it  will  nevertheless,  be  proportional  to 
the  intensity  per  square  inch,  and  can  be  used  in  the  fur- 
ther calculations.  This  intensity  is  what  may  be  allowed 
in  the  new  magnets.  Dividing  the  required  magnetism  in 
by  this  allowable  intensity,  will  give  the  area  of  cross-sec- 
tion which  the  new  magnets  must  have  in  order  not  to  be 
over-saturated,  the  quality  of  the  iron  being  supposed  to  be 
the  same.  From  this  new  area  find  the  diameter,  and  call 
it  D19  the  old  diameter  being  indicated  by  D.  It  then  re- 
mains to  find  what  the  new  ampere-turns  must  be  in  order 
to  develop  this  intensity.  From  the  formula  for  the 
value  of  /it  is  evident  that  if  the  intensities  in  two  cases 
are  the  same  the  ratios  of  the  ampere-turns  (c)  to  the 
diameter  will  be  the  same  in  the  two  cases,  that  is, 

c          c-i 

~P~~~1), 

in  which  c,  represents  the  ampere-turns  required  for  the 


Field  Magnet  Frames.  131 

new  magnets.  As  the  ampere-turns  c  of  the  old  magnets 
when  just  saturated,  and  the  old  and  new  diameters  I)  and 
Z>i  are  known,  the  value  of  the  new  ampere-turns  Cj  is 
readily  determined  ;  it  will  be 


Owing  to  some  small  errors  which  are  involved,  these 
calculations  will  not  be  exact,  but  they  will  serve  very 
well  as  a  guide. 

From  this  new  number  of  ampere-turns  and  from  the 
dimensions  of  the  old  magnets,  it  can  readily  be  calculated 
whether  the  new  magnets  must  be  made  longer,  or  whether 
they  may  be  still  shorter.  To  illustrate  this,  suppose  the 
old  magnets  showed  no  signs  of  heating  when  they  were  be- 
ing run  just  at  saturation,  developing  the  magnetism  which 
was  indicated  by  n.  This  would  show  either  that  more 
electrical  energy  may  be  used  for  the  magnet  coils  (i.  e.  for 
a  shunt  machine  thicker  wire  could  be  used,  or  for  a  series 
machine  thinner  wire),  or  that  the  heat  radiating  or  exter- 
nal surface  of  the  whole  coil  (not  of  the  wire  itself)  may 
be  smaller,  and  as  the  diameter  of  the  coil  is  larger  it 
shows  that  the  length  of  the  coil  could  be  less.  Knowing 
the  periphery  of  the  core,  the  size  of  the  wire,  and  the  num- 
ber of  ampere-  turns,  and  assuming  a  certain  thickness 
or  depth  of  coil,  the  length  of  core  necessary  to  contain  this 
wire  can  readily  be  calculated  This  subject  of  the  wind- 
ing of  magnets  and  length  of  core  will  be  further  discussed 
in  the  next  chapter. 

The  types  of  field  magnet  frames  now  in  common  use 
and  having  only  two  active  poles,  may  be  divided  into  the 
four  following  general  classes,  shown  in  figures  25,  26,  27, 
and  28,  in  which  the  coils  are  marked  ra.  They  are  classi- 
fied here  with  reference  to  the  relative  position  of  the 
magnets  proper,  their  pole  pieces  and  their  yoke  pieces. 
Any  other  modifications  of  these  general  types  are  consid- 
ered as  details  which  do  not  effect  the  general  classification. 

The  form  shown  in  figure  25,  consisting  of  a  single  U 


132 


Principles  of  Dynamo-Electric  Machines. 


magnet  with  two  coils,  is  probably  the  oldest  type.  It  was 
used  in  the  old  machines  of  Wilde  and  Pacinotti.  It  is 
the  form  used  at  present  in  the  Edison,  Hopkinson  and 
Jurgensen  machines,  and  when  the  yoke  piece  is  also 


Fig. 


Fig.  26 


Fig.  2 


.  28 


} 


covered  by  the  coils  it  is  the  form  used  in  the  Sir  Win. 
Thompson  and  the  Mather  machines.  For  the  same  amount 
of  magnetism  in  the  armature  field,  the  only  advantage  in 
covering  the  yoke  piece  with  coils,  is  a  slight  saving  of 
iron  and  of  wire,  as  it  is  evidently  a  more  economic  dis- 


Field  Magnet  Frames  133 

position  of  the  wire  and  iron,  but  it  is  accompanied  by  in- 
creased difficulties  in  winding.  The  type  shown  in  figure 
25,  is  better  adapted  for  Gramme  ring  armatures  than  for 
cylinder  armatures,  as  the  field  is  not  always  perfectly 
balanced,  being  stronger  on  that  side  of  the  armature1 
nearest  the  yoke  piece.  The  difference  may,  however,  be 
made  very  slight. 

The  more  common  type,  shown  in  figure  26,  consists  of 
two  U  magnets  with  their  like  poles  together.  For  the 
same  magnetism  in  the  armature,  the  cores  and  yoke  pieces 
need  be  only  about  half  as  large  in  cross-section,  as  they 
would  have  to  be  in  figure  25,  as  the  lines  of  force, 
shown  in  dotted  lines,  divide  evenly  between  the 
two  U  magnets.  The  quantity  of  wire  being  approxi- 
mately the  same,  the  length  of  the  cores  could  be  made 
somewhat  shorter  than  in  the  type  shown  in  figure  25, 
thus  shortening  the  path  of  the  lines  of  force  and  there- 
fore the  magnetic  resistance  offered  by  the  iron.  As  it  is 
desirable  to  have  this  magnetic  resistance  as  small  as  pos- 
sible, this  feature  of  shorter  circuits  is  an  advantageous 
one.  The  field  may  evidently  be  perfectly  balanced.  This 
type  is  the  most  common,  being  used  in  the  old  and  new 
Gramme  machines,  the  Weston,  Brush,  Siemens,  Maxim, 
Bnrgin,  Crompton,  Patterson-Cooper,  Kapp,  Jenney, 
Western  Electric,  Daft,  Ball,  Schuyler,  Clark,  Wood, 
Ileinrichs,  Knowles,  Westinghouse,  Consolidated  Company, 
and  no  doubt  numerous  others.  When  the  yoke  pieces 
are  likewise  covered  with  coils,  this  type  includes  the 
Ewell-Parker  frame. 

The  type  shown  in  figure  27  is  closely  allied  to  this  one, 
the  two  coils  of  each  |J  magnet  being  combined  together 
into  one,  and  placed  on  what  is  the  yoke  piece  in  figure  20. 
Comparing  it  with  figure  25,  it  will  be  seen  that  the  lines 
of  force  generated  by  one  coil  in  figure  27,  do  not  have  to 
pass  through  the  other  coil,  as  they  do  in  figure  25,  and 
therefore  the  cores  can  be  made  smaller,  each  coil  saturates 
only  its  own  core  and  not  that  of  the  other  coil.  If  this 


134  Principles  of  Dynamo- Electric  Machines. 

were  the  only  thing  to  be  taken  into  consideration,  the 
cores  in  figure  27  need  be  only  half  as  large  in  cross-section 
as  those  in  figure  25.  The  length  of  wire  required  will, 
however,  affect  this  dimension  on  account  of  the  limited 
length  of  the  cores  which,  in  this  type  is  dependent  on  the 
size  of  the  armature  and  other  dimensions.  Comparing  it 
with  figure  26,  it  has  the  advantage  of  having  only  two 
coils  instead  of  four  smaller  ones.  The  circuit  of  the 
lines  of  force  and  therefore  the  magnetic  resistance,  may 
also  in  many  cases  be  made  smaller  in  figure  27  than  in 
figure  26  ;  there  would  probably  also  be  a  saving  of  iron. 
The  objection  to  this  form  is,  that,  unless  the  distance 
from  the  pole-piece  to  the  coils  is  quite  large,  many  of  the 
lines  of  force  will  leak  directly  back  into  the  cores,  and 
as  these  do  not  pass  through  the  armature  they  are  wasted. 
Tins  type  is  used  in  the  McTighe,  Hopkinson,  Griscom,  and 
Joel  machines. 

The  type  shown  in  figure  28  may  also  be  considered  as 
two  |J  magnets  shaped  like  the  path  of  the  lines  of  force, 
as  shown,  with  the  coils  around  their  common  pole  pieces. 
It  has  the  same  advantage  in  having  only  two  coils,  but 
unlike  figure  27  they  saturate  each  other's  cores,  as 
in  figure  25.  It  generally  has  the  disadvantage  of 
having  a  very  long  circuit  for  the  lines  of  force,  it 
being  considerably  over  four  times  the  length  of  a  coil  ; 
this,  however,  is  partially  overcome  by  the  fact  that  the 
coils  may  be  made  thicker  and,  therefore,  shorter  in  this 
type  than  in  the  others.  It  is  like  figure  26  and  27  and 
unlike  figure  25,  in  the  feature  that  the  field  may  be  per- 
fectly balanced.  It  is  like  figures  27  in  the  fact  that  there 
generally  is  great  leakage  of  the  lines  of  force  from  the 
pole  pieces  to  the  yoke  pieces.  Among  the  machines 
using  this  type  are  the  Thomson-Houston,  Van  Depoele, 
Hochhausen,  Kapp,  and  in  general  most  multi-polar  ma- 
chines, as  for  instance,  alternating  current  machines  of  the 
ordinary  types. 

In  some  of  the  older  forms  of  machines  several  small 


Field  Magnet  Frames.  135 

parallel  magnets  with  common  pole  and  yoke  pieces  were 
used  in  place  of  one  larger  magnet.  The  best  known  ma- 
chines of  this  kind  are  the  Edison  "  Jumbo"  and  the  older 
Weston  machines.  Such  magnets  were,  however,  soon 
abandoned  by  the  makers.  They  take  up  much  more 
space  and  require  a  much  larger  amount  of  wire,  the  wire 
on  those  parts  of  parallel  coils  which  face  each  other,  be- 
ing disposed  so  as  to  neutralize  the  magnetic  effect  of  those 
parts.  This  may  be  seen  from  figure  29  in  which  three 
sucli  magnets  are  shown  in  cross-section  ;  the  current  flow- 
ing around  them  as  shown,  it  will  be  seen  that  the  parts 
of  the  coils  nearest  each  other  tend  to  neutralize  one  an- 


.,  30 


other,  as  the  neighboring  currents  are  in  the  opposite  di- 
rection, thereby  destroying  their  magnetic  effect.  These 
portions  of  the  wire  are,  therefore,  inactive  magnetically, 
being  mere  dead  resistance.  In  such  a  composite  magnet 
it  can  readily  be  shown  that  there  are  lines  of  force  be- 
tween the  magnets  running  parallel  to  them,  but  in  the 
opposite  direction  to  the  useful  lines  in  the  magnet  cores. 
These  are,  of  course,  wasted,  as  they  do  not  pass  through 
the  armature. 

It  is  often  supposed  that  two  parallel  magnets  forming 
one  U  magnet  as  in  figures  25  and  26,  are  subject  to  this 
same  objection,  and  that,  therefore,  the  coils  should  not  be 
too  near  together.  But  this  is  not  the  case,  it  is  on  the 


136  Principles  of  Dynamo-Electric  Machines. 

contrary  an  advantage  to  bring  them  as  near  together  as 
the  nature  of  the  frame  will  permit.  This  will  be  seen 
from  figure  30,  which  shows  the  cross-section,  perpendicu- 
lar to  the  cores,  of  the  two  parts  of  one  U  magnet  in  fig- 
ure 25  or  26.  It  will  be  seen  that  the  currents  in  two 
neighboring  wires  of  the  two  coils  must  be  in  the  same 
direction,  in  order  to  develop  the  two  opposite  poles  at 
the  ends  of  the  magnets.  The  currents  in  the  upper  half 
of  the  lower  coil  are,  therefore,  in  the  right  direction  to 
act  with  those  in  the  upper  coil  to  saturate  the  core  N,  and 
vice  versa.  In  other  words,  each  coil  tends  to  saturate  the 
core  of  the  other  coil,  as  described  in  another  part  of  this 
chapter.  It  is,  therefore,  an  advantage  to  bring  such 
magnets  quite  close  together,  provided  they  are  not  so 
near  as  to  allow  the  lines  of  force  to  leak  directly  from 
one  to  the  other  near  the  pole  ends. 

The  shape  of  the  pole  pieces  around  the  armature  has 
already  been  discussed  in  the  chapter  on  armatures.  The 
air  space  between  the  pole  pieces  and  the  armature  core 
should  be  made  as  large  as  possible  in  area,  and  as  thin  as 
practicable.  It  is  not  the  case,  as  is  sometimes  claimed, 
that  according  to  the  law  of  inverse  squares,  the  intensity 
of  this  field  will  be  inversely  as  the  square  of  the  distance 
between  the  iron.  That  is  a  misinterpretation  of  the  law. 
The  field  does  become  weaker  as  this  thickness  increases, 
but  as  Deprez  has  shown,  this  decrease  in  strength  is  much 
smaller  than  is  generally  supposed. 

As  described  before,  the  pole-piece  projections  should  not 
be  too  near  to  each  other,  and  they  should  not  be  too  near  to 
any  other  iron  parts  on  account  of  leakage.  They  should 
be  rounded  off  on  their  outside  edge  to  avoid  the  tendency 
to  leakage  from  the  sharp  edges.  It  should  be  remem- 
bered that  these  projections  are  often  the  most  intense 
parts  of  the  field.  It  is  probably  an  advantage  to  incline 
the  edges  of  these  projections  instead  of  making  them 
parallel  to  the  axis,  in  order  to  cause  the  wire  to  enter  the 
field  gradually  instead  of  suddenly. 


Field  Magnet  Frames.  137 

It  is  important  in  designing  frames  to  balance  the  field 
as  well  as  possible,  arid  to  avoid  having  any  abrupt  changes 
in  the  density  of  the  lines  of  force,  as  would  be  the  case,  for 
instance,  at  the  sharp  edges  of  highly  magnetized  pole-piece 
projections.  The  latter  may  produce  objectionable  self- 
induction  effects  and  may  increase  the  sparking.  In  a 
perfectly  symmetrically  wound  cylinder  armature,  as  well 
as  in  a  Gramme  armature,  an  unevenly  distributed  field 
may  not  produce  objectionable  effects,  particularly  when  the 
counter  magnetism  of  the  armature  (i.  e.,  ampere-turns  on 
armature)  is  relatively  small ;  it  is  best,  however,  to  make 
the  field  as  uniform  as  possible.  It  is  important  in  using 
unsymmetrically  wound  cylinder  armatures,  and  par- 
ticularly Gramme  armatures,  to  have  the  same  number 
of  lines  of  force  in  both  pole  pieces,  and  to  have  the 
wire^  cut  them  at  the  same  speed,  otherwise  there  may  be 
local  currents  in  the  armature,  as  described  before.  It  is, 
therefore,  necessary  to  avoid  unsymmetrical  leakage,  as,  for 
instance,  from  one  pole-piece  through  half  the  armature, 
and  returning  through  the  shaft  and  the  single  iron  bearing 
support  back  to  the  magnets ;  such  lines  are  cut  by  only 
one  side  of  the  armature. 

Accessory  iron  parts  of  the  frame  or  base  plates,  should 
be  as  far  removed  from  the  pole  pieces  as  possible,  to  avoid 
leakage.  For  the  same  reason  braces  or  bearing  supports 
should  not  be  fastened  to  pole  pieces,  or  else  should  be 
made  of  brass.  If  for  convenience  they  are  of  iron 
and  are  fastened  to  the  pole  pieces,  they  should  be 
symmetrical  and  be  separated  as  far  as  possible  from 
each  other. 


CHAPTER    VIII. 

Field  Magnet   Coils. 

ALTHOUGH  methods  have  been  suggested  for  calculating 
directly  the  size  of  the  wire  and  the  number  of  turns  in 
field  magnet  coils,  from  the  magnetism  required,  yet  until 
such  methods  have  stood  the  severe  test  of  repeated  appli- 
cation in  practice,  they  must  be  regarded  merely  as  sug- 
gestions which  may  or  may  not  be  more  trustworthy  than 
the  methods  already  in  use.  It  can  hardly  be  expected 
that  any  new  methods  based  on  calculations  only,  will  ever 
prove  to  be  more  reliable  and  sure  than  the  well  tested 
method  to  be  described,  which  is  based  on  the  actual 
performance  of  the  machine. 

There  are  a  number  of  factors  which  enter  into  such  cal- 
culations, which  are  either  unknown  or  uncertain,  and 
which  will,  therefore,  make  the  results  less  reliable.  Until 
these  quantities  are  known  any  determination  of  the  coils 
based  merely  on  calculations  without  testing  the  machine, 
will  be  approximate  only.  Among  these  uncertain  or  un- 
known quantities  are  the  magnetic  qualities  of  the  particu- 
lar iron  used,  the  effect  of  the  shape  of  the  magnetic  parts, 
the  air  space  in  the  magnetic  circuit,  the  magnetic  leakage, 
the  resistance  equivalent  of  the  self-induction  of  the  arma- 
ture and  field  magnet  coils,  the  effect  of  the  number  of 
pulsations  or  of  armature  coils,  the  Foucault  currents  in 
the  armature  coils  and  in  the  pole  pieces,  the  counter  mag- 
netism in  the  armature  commonly  called  the  magnetic  lag. 
It  is  known,  however,  that  under  certain  conditions,  easily 
obtainable  in  practice,  the  effect  of  a  number  of  these 
quantities  or  proportions  is  so  small  that  it  may  be  neglected. 
The  effects  of  some  may  be  eliminated  by  taking  as  a  basis 
of  the  calculations  the  known  results  of  a  similar  machine 

(138) 


Field  Magnet  Coils.  139 

of  the  same  general  type  and  proportions,  though  not  neces- 
sarily of  the  same  actual  size.  Recent  investigations1  in 
apparently  the  right  direction,  may  however  lead  to  the 
determination  of  certain  reliable  constants  and  relations 
which  will  materially  aid  the  engineer  in  calculating  more 
definitely  the  relations  between  the  exciting  coils  and  the 
field,  without  first  constructing  a  number  of  trial  machines. 

As  described  before,  it  is  the  electromotive-force  which 
the  machine  generates  and  maintains,  the  current  being 
dependent  only  on  this  and  on  the  total  resistance  in  cir- 
cuit. This  electromotive-force  depends  on  the  armature, 
on  its  speed  and  on  the  magnetism,  which  latter  depends 
on  the  iron  parts  and  on  the  ampere-turns.  Therefore,  if 
the  armature  and  frame  have  been  constructed  as  desciib^d, 
and  the  safe  speed  determined  upon,  the  only  other  factor 
which  remains  undetermined  and  which  may  be  made  to 
correct  any  small  errors  or  inaccuracies  in  the  determina- 
tion of  the  armature  and  frame,  is  the  winding  of  the  coils. 

This  may  be  determined  with  all  due  accuracy  by  the 
following  method  :  When  the  machine  is  completed  in  all 
its  parts  except  its  coils,  erect  it  and  place  on  the  magnet 
cores  temporary  coils  of  a  known  number  of  turns.  These 
coils  should  all  have  the  same  number  of  turns  ;  the  size 
of  the  wire  will  depend  on  the  electromotive-force  and  cur- 
rent of  the  separate  exciting  machine  or  battery  which 
must  be  used  to  supply  them  with  current  ;  it  should  be 
large  if  the  exciter  gives  a  strong  current  of  low  electro- 
motive-force, and  small  if  it  gives  a  small  current  of  high 
electromotive-force.  They  may  be  wound  on  shells  of 
brass  or  ordinary  tinned  iron,  and  should  have  a  depth  of 
winding  about  equal  to  that  which  the  final  coils  should 
have,  in  order  that  the  inaccuracies  due  to  different  thick- 
nesses of  coils  may  be  eliminated. 

The  machine  is  then  run  at  its  proper  speed  as  a  sepa- 
rately excited  machine,  the  current  for  the  coils  being  sup- 

1.  Proceedings  of  the  Society  of  Telegraph  Engineers,  November  11,1886. 
Kapp  on  characteristics  of  dynamos. 


140  Principles  of  Dynamo-Electric  Machines. 

plied  by  a  separate  dynamo  or  by  a  storage  battery,  which 
must  be  so  arranged  with  adjustable  resistances  or  other 
regulating  devices,  that  the  current  in  the  coils  may  be 
varied  at  will.  The  current  from  the  machine  is  dis- 
charged into  a  large  rheostat,  or  in  the  absence  of  such  a 
resistance  it  may  be  sent  into  a  battery  of  storage  cells  or 
into  a  circuit  of  incandescent  or  arc  lamps,  though  the 
latter  is  not  to  be  recommended,  as  the  arc  lamps  make  the 
current  too  unsteady.  The  exciting  current  is  then  varied 
and  adjusted  until  the  machine  gives  its  proper  potential 
when  the  current  has  the  required  strength,  the  brushes 
having  first  been  set  to  the  proper  position  to  avoid  spark- 
ing. When  all  the  adjustments  have  been  made  and  the 
current  and  potential  measured  to  see  that  they  have  their 
desired  values,  the  current  which  flows  through  the  exciting 
coils  should  be  measured  as  accurately  as  possible.  This 
exciting  current  is  then  multiplied  by  the  total  number  of 
turns  in  the  temporary  field  coils,  which  gives  the  number 
of  ampere-turns  that  are  required  to  excite  the  magnets  of 
the  machine  while  it  is  generating  its  proper  potential  and 
delivering  the  proper  current.  From  this  number  of  ampere- 
turns  the  proper  size  of  the  wire  and  number  of  turns  for 
the  new  coils  of  the  machine  can  readily  be  determined,  as 
will  be  described.  The  magnetism  of  a  core  depending 
only  on  the  ampere-turns,  it  is  immaterial  whether  these 
are  generated  by  many  windings  with  a  small  current  as  in 
shunt  machines,  or  by  few;  windings  with  a  strong  current 
as  in  series  machines,  or  by  a  combination  of  both  as  in  com- 
pound machines  ;  all  that  is  necessary  is  that  the  sum  of 
the  products  of  the  current  and  number  of  turns  of  wire 
remains  the  same. 

The  following  precautions  should  be  taken  in  making 
this  preliminary  test  for  determining  the  ampere-turns  : — 
The  temporary  coils  of  the  machine  when  separately  ex- 
cited should  be  connected  in  series  with  one  another,  for  if 
they  arc  in  multiple  arc  an  error  may  arise  from  a  possible  un- 
equal distribution  of  the  exciting  current  in  the  different 


Field  Magnet  Coils.  141 

coils  due  to  different  resistances.  In  the  absence  of  an  ex- 
citing machine  or  battery  the  temporary  coils  may  be  sup- 
plied with  current  from  the  machine  which  is  being  tested  ; 
they  may  then  be  wound  with  tolerably  fine  wire  and  have 
an  adjustable  resistance  in  series  with  them,  the  connections 
being  made  like  those  for  a  shunt  machine,  the  shunted 
current  being  added  to  the  main  current  as  part  of  it.  Or 
the  temporary  coils  may  be  wound  with  coarse  wire  and 
connected  as  in  a  aeries  machine,  the  adjustable  resistance 
being  in  that  case  connected  as  a  shunt  to  the  coils,  and  the 
potential  of  the  machine  being  measured  at  the  brushes  and 
not  at  the  poles,  as  some  of  the  potential  is  used  in  the  coils. 

If  the  finished  machine  is  to  be  a  series  wound  machine 
a  portion  of  the  potential  of  the  armature  is  consumed  in 
sending  the  current  through  the  magnets.  This  may  be 
as  low  as  1.5  or  2$  of  the  whole  potential  in  large  well  built 
machines,  and  as  high  as  from  15  to  20$  in  smaller  cheaper 
machines  ;  it  represents  the  percentage  of  energy  required 
in  the  magnets.  In  the  preliminary  test  for  determining 
the  ampere-turns  this  should  be  taken  into  account,  the  ex- 
citing current  being  increased  until  the  potential  of  the 
machine  is  that  which  will  be  required  for  the  magnets  in 
addition  to  that  required  for  the  external  circuit.  If  the 
finished  machine  is  to  be  shunt  wound,  a  fractional  part  of 
the  whole  current  generated  will  be  required  for  the  mag- 
nets ;  this  varies  from  1.5  to  2$  of  the  whole  current  in 
large  well  built  machines,  and  is  as  high  as  15  to  20$  in 
small  or  cheap  machines.  In  the  test  for  the  ampere-turns 
the  machine  should  therefore  be  excited  until  the  current 
is  equal  to  the  sums  of  the  main  current  and  this  shunted 
current.  In  compound  machines  both  of  these  corrections 
should  be  made,  the  whole  percentage  of  energy  being 
divided  between  the  two  sets  of  coils  according  to  their 
functions.  For  instance  if  3$  be  allowed  for  the  magnets, 
£  of  this  or  If0  of  the  potential  might  be  allowed  for  the 
series  coils,  and  f  or  2$  of  the  current  for  the  shunt  coils. 

The  magnets  should  not  be  over-saturated  except  when 


143  Principles  of  Dynamo-Electric  Machines. 

it  is  desired  to  "  steady"  the  current  by  having  the  mag- 
nets less  sensitive  to  changes  of  current  in  their  coils. 
This  applies,  of  course,  only  to  self -excited  machines  ;  in 
those  in  which  the  magnets  are  separately  excited,  as  in 
some  central  station  plants,  or  as  in  most  alternating  current 
machines,  there  is  apparently  no  reason  for  over-saturation. 
Well  built  compound  machines  should  never  be  over-satu- 
rated, nor  even  too  near  the  point  at  which  saturation  ap- 
pears to  be  complete,  as  it  is  then  difficult,  if  at  all  possible, 
to  wind  them  compound  for  a  constant  potential.  How  to 
determine,  before  winding  the  magnets,  whether  they  are 
over-saturated  by  the  required  magnetism,  will  be  set  forth 
in  a  subsequent  chapter  on  examining  machines. 

SEPARATELY    EXCITED    MACHINES. 

If  the  finished  machine  is  intended  to  have  its  magnets 
energized  by  the  current  from  a  separate  machine  the  cal- 
culation of  its  winding  becomes  very  simple.  Suppose  the 
preliminary  test  with  temporary  coils,  just  described, 
showed  that  20  amperes  were  required,  and  that  there  were 
250  turns  or  windings  on  each  coil,  the  type  of  magnet 
frame  being  that  shown  in  figure  26,  chapter  vn.,in  which 
there  are  four  coils ;  there  will  then  be  4  x  250  =  1,000 
turns,  which  with  20  amperes  gives  a  total  of  20,000  ampere- 
turns  as  a  measure  of  tne  magnetism  required.  The  num- 
ber of  ampere-turns  or  ampere- windings  thus  determined 
will  hereafter  be  represented  by  (a  W}. 

To  determine  the  winding  for  the  finished  machine  as- 
sume any  suitable  current  and  divide  this  number  of  am- 
pere-turns by  it ;  this  will  give  the  required  number  of 
windings  or  turns  of  wire,  which  again  divided  by  the 
number  of  coils  gives  the  windings  per  coil.  Any  suitably 
sized  wire  may  then  be  chosen,  which  when  wound  to  the 
required  number  of  turns  will  not  make  the  coil  too  large, 
a  good  practical  guide  for  which  is  to  make  the  depth  of 
the  coif  about  -J  the  diameter  of  the  core  when  round,  or  -J 
of  the  lesser  diameter  when  oval.  A  more  direct  method 


Field  Magnet  Coils.  143 

of  finding  the  proper  size  of  the  wire  is  to  divide  the  area 
of  cross-section  of  the  coil  space  in  square  inches  (that  is 
the  length  of  the  coil  multiplied  by  the  depth  or  thick- 
ness) by  the  number  of  turns  of  that  coil ;  this  will  give 
approximately  the  space  for  each  wire,  the  square  root  of 
which  is  the  diameter  of  the  insulated  wire  in  inches. 

The  current  for  such  magnets  should  be  chosen  with  ref- 
erence to  the  cost  of  the  wire  for  the  coils,  the  cost  of 
winding  it,  and  the  length  of  the  leads  from  the  exciting 
machine.  Considering  the  cost  of  the  wire  only,  the  current 
should  evidently  be  as  strong  as  possible  and  the  potential 
correspondingly  low,  in  order  to  increase  the  size  of  the 
wire  as  much  as  possible,  as  coarse  wire  is  cheaper  per  pound 
than  fine  wire,  and  it  may  be  assumed  that  the  weight  of 
coils  having  the  same  cross-section  of  coil  space  is  nearly 
the  same  whether  coarse  or  fine  wire  is  used.  Considering 
the  cost  of  winding,  however,  a  limit  is  soon  reached.  Al- 
though it  costs  less  to  wind  fewer  turns  of  coarser  wire 
than  many  turns  of  fine  wire,  the  cost  again  increases  when 
the  wire  becomes  so  thick  as  to  be  unmanageable.  Con- 
sidering both  of  these  factors  the  wire  should  be  chosen  as 
large  as  it  is  easy  and  cheap  to  wind.  If  the  exciter  is  at 
a  great  distance  from  the  machine  the  additional  cost  of 
the  leads,  for  a  strong  current,  may  necessitate  taking  a 
smaller  current,  and,  therefore,  more  turns  of  finer  wire  on 
the  magnets  ;  but  this  case  would  probably  occur  seldom, 
if  ever,  as  the  exciting  machine  is  generally  near  the  other. 

In  some  cases  it  may  be  preferable  to  reverse  this  method 
and  to  determine  first  the  largest  wire  which  it  is  practi- 
cable to  use>  then  find  how  many  turns  can  be  wound  into 
the  given  coil  space,  and  finally  by  dividing  this  number  of 
windings  into  the  number  of  ampere-turns  per  coil,  find 
what  the  current  must  be. 

The  case  may  occur  when  the  magnets  must  be  wound  to 
suit  a  certain  exciter  giving  a  definite  current  and  electro- 
motive force.  The  current  for  the  magnets  being  fixed, 
the  number  of  windings  are  determined  by  dividing  it  into 


144  Principles  of  Dynamo-Electric  Machines. 

the  ampere-turns  (a  TF).  The  size  of  the  wire  must  then  be 
chosen  so  that  when  wound  to  the  required  number  of  turns 
its  resistance  is  such  that  the  potential  required  is  within 
the  limits  of  the  capacity  of  the  exciter.  In  other  words, 
if  C  and  E  are  the  available  current  and  potential  of  the  ex- 
citer, the  resistance  of  the  coils  must  not  be  greater  than 

R  =  •*!.    On  account  of  the  self-induction  of  the  coils,  the 

C 

heating,  a  possible  slight  error  in  the  diameter  of  the  wire, 
or  other  factors  increasing  the  apparent  resistance,  it 

Tjr 

should  not  be  as  large  as  —  >   but  should  contain  a  "  factor 

C 

of  safety"  by  making  it,  say  15  to  20$  less. 

The  size  of  the  wire  could  be  determined  by  trial  by  cal- 

culating the  resistance  of  the   required   number  of  turns 

with  different  sized  wires,  but  the  following  method  will  be 

found  to  be  more  direct.  , 

Let  dy  represent  the  diameter  of  the  bare  wire  in  mils 

or  thousandths  of  an  inch  ; 
/m,  the  average  length  of  one  turn  in  inches  ; 
7£,  the  resistance  of  all  the  magnet  coils  in  legal  ohms  ; 
TF,  the  total  number  of  turns  on  the  magnet  coils  of 

the  finished  machine  ; 
a,  the  current  in  amperes  in  the  magnet  coils  of  the 

finished  machine  ; 
(a  TF),  the  ampere-turns  obtained  from  the  prelimi- 

nary test  ; 
6,  the  specific  resistance   (metric)  at  the  allowable 

temperature  of  the  coils. 
The  resistance  of  the  finished  coils  will  then  be 


R  =  50.13 


Field  Magnet  Coils.  145 

and  according  to  Ohm's  law, 


in  which  v  is  the  potential  in  volts  which  is  consumed  in 
the  coils.     Hence  it  follows  that 


-V 


50.13  (aW)lm0 


v 
or  assuming  6  equal  to  .0162  and  the  temperature  about 

70°  F, 



' ^ — 


in  which  it  is  necessary  merely  to  substitute  the  particular 
values,  and  reduce  it,  to  find  the  diameter  directly,  v 
should  in  this  case  be  15  to  20$  less  than  that  which  the 
exciter  will  give,  in  order  to  include  the  factor  of  safety 
above  referred  to. 

In  order  to  calculate  the  diameter  of  the  wire  by  this 
method,  it  is  necessary  to  know  the  mean  length  of  one 
winding  of  a  magnet  coil.  But  this  is  itself  dependent 
upon  the  size  of  the  finished  coil  which  however  is  not  yet 
known.  A  formula  might,  therefore,  be  deduced  which 
should  give  the  diameter  of  the  wire  directly  without  first 
knowing  the  length  of  a  winding,  but  it  has  been  found 
that  such  a  formula  is  too  complicated  to  be  of  much  use 
to  the  practical  builder  of  dynamos.  In  place  of  such  a 
complex  formula  the  following  approximate  method  will 
be  found  to  answer  equally  well,  and  to  be  much  simpler 
in  most  cases. 

If  the  depth  of  winding,  or  thickness  of  a  coil,  be  too 
great,  the  proportions  of  the  machine  will  not  be  the  best,' 
as  the  outside  layers  will  be  very  long  and  far  removed 
from  the  iron,  and  although  the  economy  in  the  iron  might 
be  great  yet  the  more  important  economy  in  copper  will 
be  poor,  and  the  iron  will  probably  be  over-saturated.  On 


146  Principles  of  Dynamo-  Electric  Mad  dues. 

the  other  hand  if  the  depth  of  winding  is  too  small,  the 
economy  of  iron  will  be  poor,  that  is,  the  magnets  will  be 
longer  and  therefore  heavier  than  is  necessary,  while  the 
economy  of  copper  will  be  good.  There  must,  therefore,  be 
a  certain  depth  at  which  the  economy  as  a  whole,  consid- 
ering both  the  iron  and  the  copper,  is  best.  Until  more 
extended  experiments  have  been  made  to  determine  more 
accurately  the  best  proportions,  it  will  be  found  to  give 
fair  results  to  make  the  depth  of  winding  about  one-third 
of  the  diameter  of  the  core,  if  round,  and  one-third  of  the 
lesser  diameter,  if  oval. 

In  order  to  find  the  mean  length  of  one  winding,  it  will 
be  sufficiently  accurate  for  all  practical  purposes  to  assume  a 
depth  of  winding  as  described,  and  from  this,  together  with 
the  dimensions  of  the  cross-section  of  the  core,  to  calculate 
the  mean  length  of  a  turn.  If  the  thickness  of  the  finished 
coil  should  then  be  found  to  be  slightly  different  from  that 
assumed,  it  will  make  very  little  difference  in  the  mean 
length,  for  in  an  actual  case  it  was  found  that  diminishing 
the  depth  as  much  as  50  per  cent,  decreased  the  mean 
length  of  one  turn  only  seven  per  cent. 

For  an  oval  coil  not  too  flat  the  lengths  of  the  inside  and 
outside  layers  are  in  the  proportion 

a  -f  b 
a-\-  b  +  l  c 

in  which  a  and  b  are  the  length  and  breadth  of  the  cross- 
section  of  the  core  and  c,  the  assumed  depth  of  winding. 
From  this  the  mean  length  of  a  turn  in  inches  will  be 


in  which  I  is  the  periphery  of  the  core  or  the  length  of  one 
turn  of  the  lowest  layer,  all  dimensions  being  in  inches. 
For  a  circular  coil  the  diameter  of  whose  core  is  J9,  the 
mean  length  of  a  turn  will  be 

t  =  7t  (D  +  c) 


Field  Magnet  Coils.  147 

which,  if  c  is  one-third  of  D,  becomes 

lm  =  4  TT  J)  =  4.189  D 

Having  thus  found  the  mean  length  of  a  turn,  there  re- 
mains only  to  substitute  its  value  in  the  above  formula  for 
d,  in  order  to  calculate  the  diameter  of  the  bare    wire, 
which  when  wound  to  the  proper  number  of  turns  will 
have  such  a  resistance  as  will  enable  the  proper  current  to 
flow  through  the  coils  when  the  exciter  has  the  potential 
stated  at  the  outset.     There  is   one   condition   however, 
which  must  be  fulfilled,  namely  that  the  actual  depth  of 
winding  should  not  exceed  too  much  that  which  was  assumed, 
as  the  resistance  may  otherwise  be  too  great.     In  general, 
if  the  actual  depth  of  winding  is  found  "to  differ  greatly 
from  that  which  has  been  assumed  it  shows  that  the  mag- 
nets are  not  proportioned  as  well  as  they  might  be.     It  is 
necessary,  therefore,  to  check  the  results  obtained,  and  at 
the  same  time  to  see  whether  the  magnet  cores  are  prop- 
erly proportioned,  by  proceeding  as  follows.     Having  cal- 
culated the  diameter  of  the  bare  wire,  as  described,  find 
what  the  outside  diameter  of  the  insulated  wire  is,  which 
may  be  done  by  winding  closely  ten  turns  around  a  man- 
drel, and  measuring  the  length  of  this  small  coil  in  inches 
and  decimals,  which  when  multiplied  by  100,  will  give  the 
outside  diameter  of  the  insulated  wire  in  mils  ;  let  this  be 
represented  by  d^    Dividing  this  into  the  length  of  a  core 
in  inches  multiplied  by  1,000  to  reduce  it  to  mils,  will  give 
the  number  of  turns  per  layer,  and  this  divided  into  the 
number  of  turns  per  coil  found  at  the  outset,  will  give  the 
number  of  layers,  from  which  the  depth  of  winding  is 
readily  found,  as  it  may  be  taken  approximately  equal  to 
the  number  of  layers  multiplied  by  di  in  mils,  and  divided 
by  1,000  to  reduce  it  to  inches. 

If  this  is  found  to  be  about  the  same  depth  of  winding 
as  that  which  was  assumed  at  the  outset,  the  core  and  coils 
are  probably  well  proportioned,  provided  other  points  such 
as  the  heating  of  the  wire  and  the  over-saturation  of  the 


148  Principles  of  Dynamo- Electric  Machines. 

core  have  been  guarded  against.  If  this  actual  depth  is 
much  greater  than  that  assumed,  it  shows  that  the  cores  or 
coil  spaces  are  probably  too  short,  and  should  be  lengthened 
to  such  an  extent  that  when  the  same  number  of  turns  are 
wound  around  the  new  core,  the  depth  of  the  coil  will  not 
exceed  about  one-third  of  the  lesser  diameter  of  the  core. 
If  this  actual  depth  is  only  slightly  greater  than  that  as- 
sumed, it  may  not  be  necessary  to  lengthen  the  cores  of 
that  particular  machine,  though  it  is  advisable  to  make  the 
change  on  the  pattern  if  it  is  to  be  used  for  other  machines  ; 
in  case  it  is  not  changed  for  that  particular  machine,  it  is 
advisable  to  make  a  new  calculation  for  the  depth  and  the 
mean  length  of  a  winding  found  from  the  first  calculation. 

If,  on  the  other  hand,  the  actual  depth  is  found  from  the 
calculations  to  be  less  than  that  assumed,  the  machine  may 
be  finished  without  any  alterations  in  the  size  of  the  core  ; 
but,  as  the  core  in  this  case  is  longer  than  it  needs  to  be,  it 
would  be  better  for  the  sake  of  economy  of  iron,  to  shorten 
the  cores  so  that  the  same  number  of  turns  on  the  shorter 
core  would  make  the  thickness  or  depth  of  winding  about 
that  which  was  assumed  ;  the  machine  would  otherwise  be 
uselessly  heavy. 

It  is  assumed  in  these  cases  that  the  magnet  frames  are 
not  over-saturated  more  than  is  desired,  and  that  the  coils 
do  not  heat  too  much  In  order  to  guard  against  over-satu- 
ration, it  is  preferable  first  to  make  the  test  for  saturation, 
which  will  be  described  in  a  subsequent  chapter,  in  order  to 
find  what  the  allowable  number  of  ampere-turns  is  in 
case  the  magnet  frame  is  over-saturated,  or  the  cores  im- 
properly proportioned.  This  test  gives  the  greatest  num- 
ber of  ampere-turns  which  should  be  used  on  that  machine. 
If  the  number  of  ampere-turns  which  are  required  to  gene- 
rate in  the  armature  the  required  potential  and  current  be 
found  to  exceed  this  limit,  it  shows  that  some  or  all  of  the 
iron  parts  of  the  machine  are  too  small  for  the  required 
magnetism,  and  that,  if  the  machine  is  to  be  properly  pro- 
portioned, the  cross-section  of  the  iron  parts,  measured  per- 


Field  Magnet  Coils.  149 

pendicularly  to  the  lines  of  force,  should  be  increased  by  a 
certain  definite  amount  which  can  be  calculated  as  described 
in  chapter  vii.  If  the  iron  is  not  over-saturated  to  a  great 
extent,  it  may  be  possible  to  finish  the  machine  without 
altering  the  size  of  the  iron  parts,  by  increasing  the  depth 
of  winding  or  the  amount  of  current  or  potential  allotted 
to  the  coils,  or  both,  but  the  machine  will  then  evidently 
not  have  the  best  proportions.  If,  on  the  other  hand,  the 
number  of  ampere-turns,  which  are  required  to  generate  in 
the  armature  the  required  potential  and  current,  be  found 
to  be  much  less  than  the  limit  obtained  from  the  saturation 
test,  no  alterations  in  the  size  of  the  iron  parts  are  neces- 
sary, but  as  this  shows  that  there  is  more  iron  than  is  essen- 
tial to  produce  the  required  results,  the  machine  is  uselessly 
large  and  heavy,  and  there  fore  has  not  the  best  proportions. 
It  would  be  preferable  to  reduce  the  cross-section  of  the 
iron  parts,  as  described  in  chapter  vii.,  in  order  that  they 
may  be  just  at  the  point  of  saturation,  or  slightly  above  it, 
if  desired,  for  the  sake  of  steadiness  of  current. 

To  guard  against  heating,  the  following  general  rules 
may  serve  as  a  guide  :  In  large  magnet  coils  the  current 
density  in  the  wire  may  be  about  .001  to  .0015  amperes 
per  square  mil  or  .00079  to. 0012  per  circular  mil1  of  cross- 
section  of  the  bare  wire,  which  is^equal  to  about  1000  to 
1500  amperes  per  square  inch,  or  1000  to  650  square  mils 
per  ampere,  or  1300  to  850  circular  mils  per  ampere. 
In  magnet  coils  made  of  fine  wire,  as  in  shunt  ma- 
chines, the  current  density  in  amperes  per  square  mil 
may  be  slightly  higher  than  in  coils  of  coarse  wire,  as  in 
series  machines.  The  size  of  the  wire  should,  therefore, 
also  be  determined  from  the  current  in  order  to  guard 
against  heating,  besides  being  calculated  from  the  formula 
first  given,  in  which  it  depends  on  the  allowable  resistance. 
Should  these  two  values  for  the  diameter  differ,  the  larger 
of  the  two  will  have  to  be  taken  in  order  that  both  the 
resistance  and  the  heating  will  not  exceed  the  limits. 

1.  A  circular  mil  is  the  area  of  a  circle  whose  diameter  is  one  mil. 


150  Principles  of  Dynamo- Electric  Machines. 

But  a  rule  of  this  kind  in  which  the  current  is  limited 
only  by  the  cross-section  of  the  wire,  although  very  simple 
in  its  application,  is  not  sufficiently  general  to  be  relied 
upon  in  all  cases.  While  it  will  give  fair  results  for  simi- 
larly proportioned  coils,  not  differing  too  much  in  the  actual 
sizes,  it  is  less  reliable  for  coils  whose  proportions  and 
dimensions  differ  very  considerably  from  those  from 
which  the  constants  were  taken,  which  were  ordinary 
oval  machine  magnets,  not  too  long,  having  a  moderate 
depth  of  winding,  and  which  were  exposed  to  the  air 
on  all  sides.  A  much  more  reliable  rule,  though  less 
convenient  and  not  always  as  simple  in  its  application,  is 
based  on  the  following  considerations  :  The  heat  in  a  coil 
is  developed  by  the  watts  or  volt-amperes  which  are  con- 
sumed in  the  coil  itself  ;  this  is  a  constant  and  continuous 
supply  of  heat  which  would  continue  to  accumulate  and 
ultimately  destroy  the  coil  if  it  were  not  continually  dissi- 
pated from  the  outside  surface  of  the  coil  as  well  as  from 
the  outside  surface  of  the  iron  parts  in  the  immediate 
neighborhood  of  the  coil.  The  dissipation  of  the  heat  will 
depend  on  the  temperature  and  on  this  exposed  or  outside 
surface,  which  for  simplicity  in  calculations  will  be  lim- 
ited here  to  that  of  the  coil  alone.  The  coil  will  therefore 
increase  in  temperature  until  the  heat  dissipated  is  equal 
to  that  generated  in  it.  In  calculating  a  coil  to  guard  it 
against  heating  too  much,  it  is  therefore  better  to  consider 
only  these  two  factors,  namely,  the  watts  consumed  in  the 
coil  and  the  outside  surface  of  the  coil.  The  density  of  the 
current  in  the  wire,  or,  in  other  words,  the  cross-section  of 
the  wire  as  compared  to  the  current,  may  be  neglected  en- 
tirely, for  it  is  evident  that  if  the  outside  surface  of  the 
coil,  and  the  number  of  watts  consumed,  be  the  same  in 
any  two  coils,  the  heating  will  be  the  same  no  mattei\what 
the  current  per  square  inch  is.  For  instance,  suppose  two 
magnet  coils  are  wound  so  as  to  have  the  same  external 
dimensions,  one  having  two  layers  and  the  other  one  layer 
of  the  same  sized  wire  ;  the  resistance  of  the  former  will 


Field  Magnet  Coils.  151 

be  about  twice  that  of  the  latter.  Now  if  the  current  is 
10  amperes  in  the  first  case  and  14  amperes  in  the  second, 
the  energy  in  watts  will  be  about  the  same  in  both  cases, 
and  as  the  external  cooling  surface  is  also  the  same,  the 
heating  will  be  the  same,  notwithstanding  that  the  current 
density  is  very  much  greater  in  the  second  case  than  in  the 
first.  The  same  result  might  be  shown  with  coils  of 
unequal  sizes  and  shapes,  having  different  depths  of  wind- 
ing. In  general,  the  current  density  may  be  greater  the 
less  the  depth  of  winding  or  number  of  layers. 

In  an  article  by  Prof.  George  Forbes,1  he  deduces  the 
following  general  laws  regarding  the  heating  of  coils  :  In 
coils  of  the  same  size  but  wound  with  wires  of  different 
diameters,  so  as  to  occupy  the  same  volume,  the  current 
must  vary  as  the  cross-section  of  the  wire,  in  order  that 
they  attain  the  same  temperature.  In  other  words,  in  such 
coils  the  current  density  in  the  two  wires  must  be  the  same. 
Another  deduction  is,  that  in  two  coils  of  similar  shape 
(the  linear  dimensions  and  the  diameter  of  wire  of  the  one 
being  n  times  that  of  the  other),  the  squares  of  the  cur- 
rents will  be  proportional  to  the  cubes  of  the  linear  dimen- 
sions when  the  heating  is  the  same.  In  other  words,  if 
one  coil  is  twice  as  large  in  all  linear  dimensions  as 
another,  and  has  wire  of  twice  the  diameter,  the  square  of 
the  current  in  the  larger  one  may  be  made  23  ==  8  times  as 
great  as  the  square  of  the  current  in  the  smaller;  that  is,  the 
current  in  the  larger  may  be  |/8  =  2.8  times  that  in 
the  smaller,  in  order  that  both  attain  the  same  tem- 
perature. 

In  the  same  article  he  gives  a  formula  which  he  states 
agrees  with  what  is  actually  found  in  practice.  From 
this  formula  the  following  practical  rules  may  be  deduced: 
A  watt  of  electrical  energy  will  be  dissipated  for 
every  800  square  centimeters  of  external  surface  of  the 
coil,  when  the  temperature  of  the  coil  is  1°  centigrade 

1.  Proceedings  of  the  Society  of  Telegraph  Engineers  and  Electricians, 
March  24,  1884. 


152  Principles  of  Dynamo-Electric  Machines. 

higher  than  that  of  the  air  ;  reducing  this  to  our  ordinary 
units,  it  is  223  square  inches  for  1°  Fahrenheit.  The  num- 
ber of  watts  which  can  be  dissipated  from  a  given  surface 
of  coil  increases  directly  with  the  temperature  which  the 
coil  may  have  above  that  of  the  atmosphere  ;  that  is,  for 
double  the  increase  in  temperature  the  watts  in  the  coil 
may  be  twice  as  great ;  or  for  the  same  allowable  temper- 
ature, it  increases  directly  as  the  surface  of  the  coil  is  in- 
creased. The  same  rules  may  also  be  stated  as  follows  : 
The  rise  of  temperature  above  that  of  the  atmosphere  will 
be  1°  centigrade  for  every  watt  and  every  800  square  cen- 
timeters of  surface  ;  or  1°  Fahrenheit  for  every  watt  and 
every  223  square  inches  of  surface.  For  the  same  surface 
it  increases  as  the  number  of  watts,  or  for  the  same  watts 
it  increases  as  the  surface  decreases.  Every  square  centi- 
meter will  dissipate  a  watt  at  800°  C.  above  the  atmos- 
here,  and  every  square  inch  will  dissipate  a  watt  at  223° 
F.  above  the  atmosphere.  The  surface  must  increase  with 
the  watts  or  decrease  as  the  allowable  temperature 
increases. 

All  these  rules  may  be  expressed  by  the  formula, 


800 

in  which  w  is  the  number  of  watts  lost  in  the  coil ;  t  is 
the  temperature  in  centigrade  degrees  above  that  of  the 
air  ;  s  is  the  outside  surface  of  the  coil  in  square  centi- 
meters. 

In  the  other  units  it  will  be 

w  =  J_    T  S  =  .004.476   T  S 
223 

in  which  T  is  in  Fahrenheit  degrees,  and  S  in  square 
inches. 

In  place  of  the  number  of  watts,  w,  may  be  substituted 


Field  Magnet  Coils.  153 


the  equivalents    G  E,  (72  -R,  or  -—  ,  in  which  (7,  E  and  R 

Ji 

represent  the  current,  electromotive  force  and  resist- 
ance, respectively,  of  the  coil,  the  resistance  being  that  at 
the  temperature  to  which  the  coil  may  be  raised.  For  in- 
stance, substituting  G  E  for  w  and  reducing,  gives 

a-      TS 

'    223  M 

• 

which  gives  directly  the  greatest  current  which  can  be 
used  in  the  magnet  coils  of  a  shunt  machine  having  a  cer- 
tain potential,  in  order  that  they  do  not  heat  above  an 
allowable  temperature  ;  this  current  can  therefore  be  cal- 
culated even  before  the  winding  has  been  determined,  as 
it  is  independent  of  the  number  of  turns  or  the  resistance, 
if  only  the  size  of  the  external  surface  is  known  approxi- 
mately ;  this  maximum  current  should  never  be  exceeded. 
Numerous  other  useful  formulae  can  be  deduced  in  the 
same  way.  For  instance,  if  a  maximum  rise  of  tempera- 
ture of  80°  F.  above  that  of  the  air  is  allowed  for  the  mag- 
nets, the  greatest  allowable  current  will  be 

C  =  .36  -f  , 

or  if  the  resistance  instead  of  the  electromotive  force  is 
known, 


G  =  .60    A/    jp 

Similarly  for  series  wound  magnets  in  which  the  current 
is  known,  the  formula  will  give  the  greatest  resistance 
which  may  be  given  to  the  coils  in  order  not  to  heat  too 
much  ;  thus, 


223 


154  Principles  of  Dynamo-Electric  Machines. 

and  if  the  limit  is  a  rise  of  80°  F.,  the  greatest  allowable 
resistance  will  be 

X  =  -36  -^. 

As  these  formulae  give  the  resistance  at  the  higher  tem- 
perature, allowance  should  be  made  for  this  increase  in 
determining  the  resistance  cold.  Every  degree  Fahrenheit 
increases  the  resistance  about  two  tenths  of  one  per  cent., 
(.2  per  cent.)  therefore  at  80°  above  the  normal  tempera- 
ture it  would  be  1  -f  (.002  X  80)  =  1.16  times  as  great, 
and  should  therefore  be  divided  by  this  number. 

The  same  formula  may  be  used  equally  well  to  find  what 
the  surface  of  a  coil  should  be.  Thus, 

223  w 
~T~ 

which  for  a  rise  of  80°  F.  is 

S  =   2.8  w. 

As  the  number  of  watts  to  be  allotted  to  the  magnets  is 
generally  known  approximately  at  the  outset,  this  formula 
enables  one  to  determine  about  what  the  least  surface  of 
the  coils  should  be,  and  as  the  cross-section  of  the  core  is 
known,  and  by  assuming  a  depth  of  winding  of  about  % 
the  diameter,  the  least  length  of  the  core  can  readily  be 
determined.  This  should,  however,  be  used  only  as  a 
guide,  as  the  dimensions  may  have  to  be  larger  on  account 
of  the  limited  resistance  of  the  coils. 

In  general,  when  the  coils  are  short  and  have  a  great 
depth  of  winding,  or  when  a  relatively  large  percentage  of 
the  total  energy  of  a  machine  is  consumed  in  compara- 
tively small  coils  (which  is  frequently  the  case  in  small, 
cheap,  or  poorly  proportioned  machines),  the  heating  limit 
will  determine  the  size  of  the  coils,  as  the  size  calculated 
from  the  allowable  resistance  would  give  smaller  values, 
which  would  cause  too  much  heating.  On  the  other 


Field  Magnet  Coils.  155 

hand,  wa«n  the  coils  are  long  and  have  only  a  few  layers, 
or  wLen  the  percentage  of  energy  in  the  magnets  is  com- 
paratively small  (which  is  frequently  the  case  with  large 
well  proportioned  machines),  the  sizes  will  depend  chiefly 
on  the  allowable  resistance,  as  the  heating  limit  will  gen- 
erally give  smaller  values.  But  these  are  only  general 
rules  $  they  may  vary  greatly  in  special  cases. 

Recurring  to  the  formula  for  calculating  the  smallest 
allowable  diameter  of  the  wire  directly  from  the  required 
number  of  ampere-turns,  namely  : 


.875  (aW)  lm 


it  is  evident  that  its  application  is  not  limited  to  the  par- 
ticular case  "before  stated,  but  that  in  general,  it  may  be 
used  in  any  case  if  only  the  ampere-turns,  mean  length  of 
one  winding,  and  the  potential  at  the  ends  of  the  coils, 
are  known  ;  as  the  formula  does  not  contain  the  current  as 
one  of  its  factors,  it  is  not  necessary  to  know  this  current 
to  calculate  the  diameter  of  the  wire.  For  instance,  if 
only  the  potential  of  the  exciter,  and  not  its  current,  is 
fixed,  or  if  a  number  of  machines  are  to  be  excited  in  com- 
mon by  one  exciter,  the  fields  being  all  in  multiple  arc 
with  one  another,  the  diameter  of  the  wire  can  be  calcula- 
ted without  first  knowing  the  current.  After  determining 
the  diameter  of  the  wire  any  suitable  current  may  then 
be  chosen,  and  when  divided  into  the  number  of  ampere- 
turns  required,  will  give  the  number  of  windings  of  this 
wire.  But  this  current  is  not  only  limited  by  other  con- 
siderations to  a  certain  range  of  values,  which  range  may 
sometimes  be  quite  great,  but  in  most  cases  there  will 
be  one  particular  current  which  will,  considering  every- 
thing, be  the  most  desirable  in  each  particular  case. 

The  principal  considerations  which  limit  the  choice  of 
this  current,  are  the  following.  Suppose  a  very  great  cur- 
rent is  used,  and,  therefore,  a  correspondingly  small  num. 


156  Principles  of  Dynamo-Electric  Machines. 

ber  of  windings,  then,  as  the  potential  remains  the 
same,  the  energy  in  the  coils  will  increase  with  the 
current  and  may  exceed  the  heating  limit,  as  the  radiating 
surface  is  fixed  by  the  known  dimensions  of  the  coil.  The 
greatest  allowable  current  should,  therefore,  be  based  on 
the  heating  limit  and  may  be  calculated  from  the  surface 
of  the  coil  and  the  potential,  by  one  of  the  heating  formulae 
before  given.  But  besides  this  heating  limit,  it  may  be 
restricted  to  a  still  smaller  value  by  the  desired  efficiency 
of  the  machine,  for  it  is  evident  that  if  the  energy  lost  in 
the  magnet  coils  be  relatively  great  the  efficiency  of  the 
machine  will  be  poor.  To  calculate  the  current  from  this 
limitation,  determine  how  much  energy  in  watts  may  be 
allotted  to  the  coils  and  divide  this  by  the  known  poten- 
tial, thus  giving  the  current.  The  amount  of  energy  to  be 
used  in  the  coils  will  vary  greatly  with  different  machines; 
in  large  well  built  machines  it  may  be  made  as  low  as  1.5 
to  2$  of  the  whole  output,  while  in  small,  cheap  ma- 
chines it  often  is  as  high  as  10  to  20^.  In  large  well 
proportioned  machines  this  is  the  most  important 
limitation  for  the  current,  provided,  of  course,  that 
it  does  not  exceed  the  heating  limit  as  before  de- 
scribed. As  an  increase  of  the  current  is  accompanied  by 
a  nearly  proportionate  decrease  in  the  quantity  of  wire,  and 
therefore,  also  in  its  cost,  it  is  evident  that  when  cheapness 
of  construction  is  of  more  importance  than  efficiency,  the 
current  should  be  taken  as  great  as  the  heating  limit  will 
allow  in  order  to  economize  wire,  while  if  efficiency  is  the 
chief  consideration  the  current  should  be  as  small  as  prac- 
ticable. The  smallest  value  which  the  current  may  have 
depends  on  the  greatest  amount  of  space  which  the  coil 
may  occupy  and  on  the  cost  of  the  wire.  If  the  current 
be  taken  too  small  the  number  of  turns  required  to  give 
the  necessary  ampere-turns,  may  be  so  great  as  to  occupy 
more  space  than  is  allowed  for  the  coil ;  furthermore,  the 
depth  of  winding  may  then  be  so  great  that  the  mean 
length  of  one  winding  will  become  much  greater  than 


Field  Magnet  Coils.  157 

what  was  assumed,  and  consequently  the  resistance  of  the 
coil  will  be  too  great  to  take  its  required  current  at  the 
given  potential.  This  minimum  value  of  the  current  can 
readily  be  ascertained  by  calculating  how  many  turns  of  the 
wire  determined  from  the  formula  may  be  wound  in  the 
space  allotted  for  the  coil,  and  dividing  this  number  of 
turns  into  the  required  ampere-turns,  thus  giving  the  cur- 
rent. It  may  be  preferable  in  some  cases  not  to  decrease 
the  current  to  this  smallest  value  on  account  of  the  expense 
of  the  wire.  As  the  formula  for  calculating  the  diameter 
of  the  wire  gives  that  value  which  the  wire  must  have  to 
enable  at  least  the  required  number  of  ampere-turns  to  be 
generated,  it  follows  that  this  is  the  smallest  diameter 
which  the  wire  may  have  ;  any  larger  size  than  this  may 
be  used  without  fear  of  increasing  the  resistance  by  the 
increased  mean  length  of  one  winding  due  to  the  greater 
depth  of  winding. 

From  these  considerations  it  will  be  seen  that  there  is 
no  fixed  rule  for  determining  the  winding  of  the  coils,  but 
that  there  are  certain  limiting  conditions  differing  in  their 
relative  importance  in  different  machines,  which  give  the 
designer  some  latitude  in  selecting  the  winding.  As  all 
these  limitations  can  be  readily  calculated  as  described, 
from  the  known  data,  the  most  direct  method  of  determin- 
ing the  winding  of  the  coils  appears  to  be,  to  calculate  the 
limiting  values  for  the  current,  the  number  of  windings 
and  the  diameter  of  the  wire,  and  then  to  choose  such 
values  within  these  limitations  as  will  best  meet  the 
desired  requirements  of  cheapness  in  first  cost  on  the  one 
hand,  and  good  efficiency  on  the  other.  For  instance, 
from  the  formula  determine  the  smallest  diameter  which 
the  wire  can  have  ;  from  the  heating  limit,  the  surface  of 
the  coil  and  the  potential,  determine  the  greatest  current ; 
from  the  desired  efficiency  determine  again  the  greatest 
current ;  from  the  coil  space  and  least  diameter  of  wire 
determine  the  greatest  number  of  windings  permissible  ; 
from  these,  the  best  proportions  will,  in  most  cases,  be- 


158  Principles  of  Dynamo-Electric  Machines. 

come  self  evident.  Should  some  of  the  limiting  values 
be  found  to  overlap,  as  for  instance,  if  the  smallest  allow- 
able current  should  be  found  to  be  greater  than  the  maxi- 
mum, it  shows  that  the  conditions  cannot  all  be  met  by 
those  proportions  of  the  machine. 

To  illustrate  this  by  an  actual  case,  let  it  be  required  to 
determine  the  winding  for  a  certain  machine  which  gene- 
rates 50  amperes  and  100  volts.  The  exciter  also  has  100 
volts,  which  is  equivalent  to  assuming  the  first  to  be  a 
self-exciting  shunt  machine.  The  frame  is  of  the  type 
shown  in  figure  26  with  four  coils.  The  cores  are 
oval,  the  cross-section  being  9x3  inches,  having  a  periphery 
of  20  inches  ;  length  of  cores  10  inches.  From  the  test 
with  temporary  coils  it  was  found  that  15,000  ampere- 
turns  were  required  to  excite  the  magnets  while  the  ma- 
chine was  generating  the  required  potential  and  current. 
To  calculate  the  smallest  diameter  which  the  wire  may 
have  it  is  necessary  to  find  the  mean  length  of  one  turn. 
Assuming  a  depth  of  winding  of  about  one-third  of  3  =  1 
inch,  the  mean  length  lm  will  be,  from  the  formula 

Zm  =  l+?JLl  =  23.3  inches. 
a-\-b 

The  diameter  of  the  wire  will  therefore  be,  assuming  a 
factor  of  safety  of  20$  by  making  v  =  80  instead  of  100, 


.875  (aW)  lm 
d=\l  —      -=61.8  mils. 


V 


The  nearest  gauge  number  corresponding  to  this  is  No. 
14  B.  &  s.  which  has  a  diameter  of  64  mils.  This  is 
therefore  the  smallest  wire  which  can  be  used,  and 
is  entirely  -independent  of  the  current  which  may  be 
chosen. 

To  determine  next  the  greatest  current  from  the  heating 
limit,  it  is  necessary  to  know  the  surface  of  the  coils.  The 
lengths  of  the  outside  and  inside  windings  will  be  in  the 
proportion 


Field  Magnet  Coils.  159 

a-|-&4-4c_9-j-3-|-4__    16  _      4 
a  +  b  9+3          "  ~12  ~    ~3~ 

in  which  a,  and  b,  are  the  length  and  breadth  of  the  oval, 
and  c,  the  assumed  depth  of  winding,  which  in  this  case  is 
one  inch.  The  inside  length  being  from  measurement  20 

inches,  the  outside  will  be  X  20  =  26.7  inches  ;  and  as 

•  3 

the  coils  are  10  inches  long,  all  four  will  have  a  total  sur- 
face of  26.7  X  10  X  4  =  3068  square  inches.  From  the 
heating  formula  the  greatest  permissible  current,  allowing 
a  rise  of  80°  F.,  will  be,  E  being  100  volts, 

C  =  .36    *   =  '36  X  1068    =  3.84  amperes. 
E  100 

This  should  not  be  exceeded. 

To  ascertain  next  the  greatest  current  considering  the 
efficiency,  determine  what  percentage  of  energy  may  be 
allotted  to  the  magnets  ;  as  the  machine  is  not  large,  six  to 
seven  per  cent,  should  not  be  exceeded  ;  assuming  the  lat- 
ter gives  3.5  amperes  as  the  greatest  allowable  current  for 
that  efficiency. 

Finally,  determine  the  least  current  or  greatest  number 
of  windings  from  the  coil  space.  A  No.  14  B.  &  s.  wire  has 
an  outside  diameter  of  about  88  mils.  As  the  coils  are  10 

inches  long,  there  will  be  10  X  1QOQ  —114  turns  per  layer, 

88 

and  if  the  greatest  allowable  depth  of  winding  is  about 

one  inch  there  will  be  — ^ =  11.4  or  about  12  layers 

88 

making  114X12X4  =  5472  windings  on  all  four  mag- 
nets. This  gives  for  the  least  current =  2.75  am- 

5472 

peres,  as  there  must  be  15,000  ampere-turns.  This  current 
is  considerably  smaller  than  that  obtained  from  the  heating 
limit. 


160  Principles  of  Dynamo-Electric  Machines. 

Summarizing  these  results,  if  the  coils  are  to  be  wound 
as  cheaply  as  possible,  without  regarding  the  efficiency,  3.8 
amperes  is  the  greatest  permissible  current.  This  gives 

15000  =  3950  windings  of  No.    14  wire.      On  the  other 
3.8 

hand  if  the  best  efficiency  is  desired,  the  least  possible  cur- 
rent is  2.75  amperes,  and  5472  windings  of  the  same  wire. 
As  the  cost  of  the  wire  of  the  same  size,  is  approximately 
proportional  to  the  number  of  turns,  it  will  in  the  second 

case  be  =  1.4  times  as  great  as  in  the  first  case,  while 

3950 

on  the  other  hand,  the  percentages  of  energy  in  the  coils 
will  be  7.6  in  the  first  and  5.5  in  the  second  case.  It  is 
obvious  that  there  would  be  no  advantage  in  using  a  larger 
wire  than  No.  14,  as  the  only  apparent  gain  would  be  to 
reduce  the  resistance,  thereby  increasing  the  current  and 
therefore  decreasing  the  required  number  of  turns  or  the 
length  of  the  wire  ;  but  this  current  would  be  greater  than 
3.8  and  would  therefore  increase  the  rise  of  temperature  to 
more  than  80°  F.  which  was  assumed  to  be  the  safe  limit. 
On  the  other  hand  if  a  smaller  wire  than  the  one  given  by 
the  formula  were  used  the  resistance  would  be  so  high  that 
it  would  not  be  possible,  with  the  same  factor  of  safety,  to 
generate  the  required  number  of  ampere-turns  in  the 
limited  coil  space  and  with  the  given  potential. 

If  coil  spaces  of  the  same  dimensions  be  wound  full  with 
wires  differing  in  size  in  each  case,  the  coils  will  have  the 
following  relative  properties  if  the  space  occupied  by  the 
insulation  is  not  considered.  If  d  represents  the  diameter 
of  the  wire,  the  number  of  turns  will  be  proportional  to 

— ,  that  is,  for  a  wire  having  twice  the  diameter  the  num- 
ber of  turns  will  be  one-quarter  as  great  as  before,  or  with 
half  the  diameter  they  will  be  four  times  as  great  ;  the  re- 


Field  Magnet  Coils.  161 

sistance  of  the  coils  will  be  proportional  to  — —.    If  the  same 

number  of  ampere-turns  are  to  be  generated  in  each  of 
these  coils,  the  current  must  be  proportional  to  c?2,  and  the 
electromotive  force  required  to  generate  this  current  will 

be  proportional  to  — -  or  to  the  number  of  turns  ;  the  energy 

Cl 

in  watts  will  then  be  the  same  in  all  the  coils,  and  as  the 
surface  is  the  same,  the  temperature  to  which  they  will  be 
raised  by  this  current  will  also  be  the  same.  If  instead  of 
having  the  same  number  of  ampere-turns  these  coils  be  sub- 
jected to  the  same  electromotive  force,  as  would  be  the  case, 
for  instance,  in  different  coils  for  the  same  shunt  machine, 
the  current  which  will  flow  through  them  will  be  propor- 
tional to  d\  and  the  ampere-turns  will  therefore  be  pro- 
portional to  d 2 ;  the  energy  in  watts  being  approximately 
equal  to  the  current  multiplied  by  the  electromotive  force, 
will  then  be  proportional  to  the  current  (the  electromotive 
force  being  the  same)  and  will  therefore  be  proportional 
to  c?4,  or  to  the  square  of  the  number  of  ampere-turns  ;  as 
the  surface  is  the  same,  the  temperature  will  also  be  pro- 
portional to  e?4,  or  to  the  square  of  the  number  of  ampere- 
turns  ;  furthermore,  the  energy  required  per  ampere-turn 
or  per  line  of  force  will  be  proportional  to  d 2.  From  this 
it  will  be  seen  that  when  subjected  to  the  same  electro- 
motive force,  any  number  of  ampere-turns  can  be  genera- 
ted in  a  limited  coil  space  by  merely  making  the  diameter 
of  the  wire  great  enough,  but  the  heating  of  the  coil  will 
thereby  be  increased  very  rapidly  so  that  a  practical  limit 
is  soon  reached,  which  may  be  determined  from  the  heat- 
ing formulae  ;  furthermore,  the  cost  of  generating  the  same 
amount  of  magnetism  will  increase  as  the  square  of  the 
diameter,  thus  reducing  the  efficiency.  If  instead  of  hav- 
ing the  ampere-turns,  or  the  electromotive  force  the  same 
in  such  coils,  they  have  the  same  current  passed  through 
them,  as  for  instance  in  different  coils  for  the  same  series 


162  Principles  of  Dynamo-Electric  Machines. 

machine,  they  will  have,  the  following  relative  properties  : 

the  ampere-turns  will  be  proportional  to  —  or  to  the  num- 

d* 

ber  of  turns  ;  the  electromotive  force  consumed  by  the 
current  will  be  proportional  to  their  resistance,  and  there- 
fore to  — —  ;  as  the  current  is  the  same,  the  energy  re- 
quired and  therefore  also  the  temperature  will  be  propor- 
tional to  the  electromotive  force,  or  to  the  resistance,  or  to 

—  ;  the  energy  required  per  ampere-turn  or  per  line  of 

Cv 

force,  thus  representing  the  cost  of  generating  the  mag- 
netism, will  be  proportional  to  _.  What  was  said  re- 

d 

garding  an  increase  of  diameter  of  wire  for  shunt  machine, 
applies  therefore  equally  well  to  a  decrease  of  diameter  for 
series  machines. 

From  this  it  will  be  seen  that  it  is  desirable  in  many 
cases  (for  instance  in  shunt  machines)  to  find  the  smallest 
diameter  which  the  wire  may  have.  The  formula,  men- 
tioned above,  for  calculating  the  diameter  gives  this  mini- 
mum value  under  the  given  conditions,  namely,  that  its  re- 
sistance shall  be  such  as  to  enable  the  required  number  of 
ampere-turns  to  be  generated  with  the  limited  potential, 
and  with  the  given  mean  length  of  one  turn.  While  a 
smaller  diameter  than  this  would  make  the  resistance  too 
high  for  the  necessary  current  and  therefore  render  the 
coil  useless,  a  larger  diameter  could,  of  course,  be  used, 
provided  it  is  not  limited  by  other  considerations. 

It  may  sometimes  require  less  calculation  to  determine 
the  winding  indirectly  by  trial  calculations  by  assuming 
different  sizes  for  the  wire  and  calculating  the  resistance 
and  number  of  windings  for  each  case  to  find  whether  the 
required  number  pf  ampere-turns  can  be  generated  in  them. 


Field  Magnet  Coils.  163 

While  such  a  method  may  or  may  not  be  shorter  it  does 
not  give  the  limiting  values  between  which  the  designer 
can  choose  the  one  best  suited  to  the  case,  nor  does  it  in- 
dicate whether  the  best  proportions  have  been  arrived  at, 
or  whether  any  changes  in  the  machine  are  advisable,  or 
whether  it  is  at  all  possible  to  meet  the  requirements.  The 
direct  method  will  therefore  in  most  cases  be  found  to  be 
the  shorter. 

It  is  assumed  that  the  dimensions  of  the  frame,  including 
the  length  and  cross-section  of  the  cores,  have  been  determ- 
ined as  correctly  as  possible  from  the  amount  of  magnet- 
ism required,  prior  to  the  determination  of  the  winding  of 
the  coils.  The  calculated  values  of  the  diameter  of  the 
wire  and  of  the  necessary  depth  of  winding,  can  therefore 
not  be  used  to  construct  the  cores  from;  they  will  however 
be  found  to  be  a  convenient  and  reliable  means  of  ascer- 
taining whether  the  proportions  of  the  cores  and  windings 
are  the  best,  and  if  not,  in  what  way  they  might  be 
improved. 

From  the  practical  limitation  to  some  of  the  proportions 
of  the  coils  and  from  the  general  nature  and  size  of  the 
machine,  it  will  readily  be  seen  whether  the  calculated 
diameter  of  the  wire  is  relatively  small  or  large.  Provid- 
ing that  the  heating  limit  is  not  exceeded,  and  that  the 
cores  are  not  over-saturated  more  than  is  intended,  then  a 
relatively  small  value  for  the  diameter  of  the  wire  indi- 
cates that  either  the  ampere-turns  are  relatively  small  or 
that  the  potential  is  relatively  great  for  those  coils  ;  in 
such  cases  a  larger  wire  may  be  used  if  desired,  provided 
the  coil  space  is  not  too  small,  external  resistance  being 
added  in  case  of  a  shunt  machine  to  reduce  the  current  to 
its  former  value.  If  on  the  other  hand,  the  calculated 
diameter  is  relatively  large,  it  indicates  that  either  the 
magnet  cores  or  the  ampere-turns  are  relatively  large,  or 
that  the  potential  is  relatively  small  for  those  coils. 

Similar  deductions  can  also  be  made  from  the  actual 
depth  of  winding  as  determined  from  the  diameter  of  the 


164  Principles  of  Dynamo-Electric  Machines. 

wire,  in  distinction  from  the  assumed  depth.  If,  as  before, 
the  heating  limit  is  not  exceeded,  and  the  cores  are  mag- 
netized to  the  desired  degree  of  saturation,  then  if  the 
depth  is  found  to  be  very  small  it  indicates  that  the  cores 
are  too  long,  while  if  it  is  very  great  it  indicates  that  they 
are  overworked,  being  too  short.  It  is  understood,  of 
course,  that  the  depth  of  winding  need  not  be  limited  in  all 
cases  to  a  certain  proportion  of  the  diameter  of  the  core, 
the  rule  given  above  to  this  effect  is,  as  described,  to  be 
used  merely  as  a  general  guide  in  order  to  enable  the  mean 
length  of  one  turn  to  be  calculated  approximately.  In 
most  cases  it  will  no  doubt  be  found  to  be  a  proper  pro- 
portion, but  there  may  be  other  considerations,  such  as  the 
heating,  the  limited  coil  space  or  the  length  of  core,  which 
may  necessitate  a  greater  or  less  depth  than  that  assumed; 
if  such  is  the  case  it  will  be  shown  by  the  calculations. 

Similarly  if  it  is  found  that  the  energy  required  for  ex- 
citing the  magnets  is  relatively  great,  the  magnets  are 
poorly  proportioned  or  overworked,  while  if  the  energy  re- 
quired is  found  to  be  relatively  small  it  indicates  that  the 
magnets  are  not  used  to  their  full  capacity,  and  might 
therefore  be  made  smaller.  Such  general  and  obvious  de- 
ductions from  the  proportions  of  the  coils  will  often  serve 
as  a  guide  and  will  be  particularly  applicable  for  recon- 
structing and  standardizing  machines,  and  in  designing 
from  poorly  proportioned  machines  as  models. 

In  all  these  cases  it  has  been  assumed  that  the  potential 
at  the  ends  of  the  magnet  coils  was  a  fixed  and  limited 
amount,  and  that  a  choice  of  the  current  necessary  to  meet 
the  requirements  was  left  to  the  designer  ;  these  are  the 
conditions  for  shunt  machines.  If  on  the  other  hand  the 
current  is  limited  to  a  certain  fixed  value,  as  in  series  naa- 
chines,  while  the  potential  consumed  by  the  coils  may  be 
made  anything  that  is  required,  the  calculations  of  the  coils 
from  the  ampere-turns  is  slightly  different.  The  current 
and  the  ampere-turns  being  known  at  the  outset  the  num- 
ber of  turns  is  their  quotient.  The  diameter  of  the  wire 


Field  Magnet  Coils.  165 

is  then  determined  from  the  following  limiting  conditions  : 
the  limited  coil  space,  the  heating  limit,  and  the  desired 
efficiency  of  the  magnets.  The  first  may  be  determined 
approximately  by  multiplying  the  length  of  one  coil  by  the 
allowable  depth  of  winding,  both  in  inches,  and  dividing 
this  by  the  number  of  windings  in  one  coil,  the  square  root 
of  this  quotient  is  the  diameter  in  inches  including  insula- 
tion ;  this  is  the  maximum  limit.  The  heating  limit  may 
be  determined  by  finding  the  greatest  allowable  potential 
from  the  surface  of  the  coils  and  the  current,  in  the  formula 
already  given,  namely 


which  is  then  compared  with  the  potential  obtained  from 
the  desired  efficiency  ;  this  latter,  as  described  before, 
should  be  a  certain  percentage  of  the  total  voltage  of  the 
machine.  From  these  two  values  the  proper  potential  can 
then  be  chosen  which  will  best  meet  the  desired  conditions 
of  greatest  cheapness  of  construction  on  the  one  hand 
(limited  by  the  heating),  or  of  best  economy  of  energy  on 
the  other  hand  (limited  by  the  efficiency).  The  diameter 
is  then  determined  from  this  potential  by  the  formula 
already  given.  This  will  give  the  smallest  limiting  value 
of  the  diameter.  Or  the  diameter  may  be  calculated 
directly  for  each  of  these  two  potentials  giving  two  mini- 
mum values  from  which  together  with  the  maximum  limit 
above  given,  the  proper  diameter  can  readily  be  chosen. 

It  is  assumed  herein  that  a  machine  is  to  be  constructed 
which  will  generate  a  certain  desired  electromotive  force 
and  current.  If,  as  is  no  doubt  sometimes  the  case,  the 
machine  is  built  by  guessing  at  the  proportions  and  then 
running  it  to  "  see  what  it  will  give  "  such  calculations  as 
those  described  will  not  be  necessary.  But  even  in  such 
cases  the  proper  calculations  made  from  the  results  of  a 
test,  may  show  whether  the  machine  is  running  at  its  best 
or  whether  and  how  it  could  be  improved  in  any  of  its 
parts. 


166  Principles  of  Dynamo-Electric  Machines. 

Besides  the  formulae  already  given,  the  following  may 
sometimes  be  of  use,  especially  as  a  check  to  determine  the 
correctness  of  other  calculations.  Using  the  same  letters 
and  units  as  before,  the  resistance  of  the  coils  will  be 

E=    875   Wl"    -3'5Wl» 
d*  4d* 

at  about  70°  F.  ;  every  degree  Fahrenheit  increases  the  re- 
sistance about  two-tenths  of  one  per  cent.  (.2  per  cent.). 
If  the  calculated  resistance  is  found  to  be  too  great,  owing 
to  slight  inaccuracies  in  the  diameter  or  assumed  conduc- 
tivity of  the  copper,  or  other  unavoidable  causes,  the 
diameter  can,  if  the  coil  space  permits,  be  made  slightly 
larger  without  fear  that  the  consequent  increased  length 
will  again  increase  the  resistance,  as  the  latter  will  increase 
only  slightly  with  a  greater  mean  length  of  one  turn,  while 
it  will  be  diminished  greatly  by  a  slightly  larger  diameter. 
The  potential  lost  in  the  coils  is  equal  to 


d*  4  d* 

The  total  length  of  wire,  in  feet,  for  all  the  coils  may  be 
calculated  from  the  formulae 

=  .0833   Wlm. 

If  e  is  the  length,  in  inches,  of  one  coil,  dlf  the  outside  di- 
ameter of  the  wire  in  mils  including  insulation,  and  n,  the 
number  of  coils,  the  number  of  turns  per  layer  in  one  coil 
will  be 

1000  e 

~^T   ; 

TF" 

the  number  of  turns  per  coil  being  —  and   the  depth   of 

n 

the  winding  being  represented  by  c  in  inches,  the  number 
of  layers  will  be 

Wd,     or  1000  c 
1000  e  n         ~~o~ 


Field  Magnet  Coils.  167 

or    the    thickness    c,    of    the    coil    in    inches     will    be 
approximately 


1,000,000  e  n 

The  figures  occurring  in  the  formulae  for  the  heating  of 
coils  are  based  on  the  constants  given  by  Prof.  George 
Forbes  in  the  paper  before  referred  to.  But  as  these  con- 
stants may,  and  undoubtedly  do,  vary  somewhat  for  diff- 
erent machines  owing  to  their  different  general  outlines, 
the  ventilation,  air  blast  from  armature,  exposed  surfaces, 
etc.,  it  is  best  whenever  practicable  to  determine  these 
constants  for  the  particular  style  of  machine,  or  for  one 
similar  in  its  cooling  properties.  This  may  readily  be  done 
by  measuring  the  outside  surface  St  of  the  coils,  finding 
their  temperature  T,  in  degrees  Fahrenheit,  after  a  long 
steady  run  with  full  load,  and  measuring  the  watts  of  en- 

ergy 10,  which  are  consumed  in  the  coils.     Then 


T 
2  o 

be  the  constant  to  take  the  place  of  ^  in  the  formula 


w 


From  this  all  the  other  heating  formulae  can  readily  be  de- 
duced by  substituting  for  w,,  its  equivalents  in  terms  of 
current,  electromotive  force  or  resistance.  All  heating 
formulae  are  obviously  only  approximately  correct  owing 
to  the  greatly  varying  conditions,  but  if  the  limiting 
temperature  is  not  taken  too  high,  a  moderate  difference 
between  the  actual  and  the  calculated  heating  will  not 
be  of  any  great  importance. 

The  following  formula  by  Brough,  may  sometimes  be 
of  use  in  calculating  coils  having  a  limited  resistance.  It 
is  for  determining  the  diameter  d  of  the  bare  wire  which, 
when  insulated  and  wound  to  fill  a  limited  coil  space,  will 
have  the  required  resistance  E.  It  is  for  circular  coils 
only. 


168  Principles  of  Dynamo-Electric  Machines. 


in  which  £  is  the  radial  thickness  of  the  insulation  or  half 
the  increase  in  diameter  due  to  insulation,  e  is  the  length 
of  the  coil  space.  A,  the  outer  diameter  of  the  circular 
coil,  a  the  inner  diameter,  R  the  resistance  of  one  coil,  and 

1C,  the  resistance  of  a  piece  of  wire —  units   long    and   one 

4 

unit  in  diameter.  All  dimensions  must  be  in  the  same 
units,  either  mils  or  inches.  If  they  are  all  in  inches,  then 
JTis  about  .000000687.  For  oval  coils,  which  are  by  far 
the  more  common  in  dynamos,  the  writer  has  deduced  the 
following  similar  formula  in  which,  however,  the  quantities 
are  given  in  the  usual  units,  namely,  d  and  i  in  mils,  and 
the  coil  dimensions  (c,  depth,  e,  length  and  lm  mean  length 
of  one  turn)  in  inches.  It  can  be  used  for  both  circular  or 
oval  coils. 


d  = 


875000  c   e  lm 


In  winding  the  coils  it  is  very  important  to  insulate  the 
wire  carefully  from  the  core  and  at  the  ends  of  the  coil. 
One  or  two  layers  of  strong,  tough  and  flexible  cardboard 
about  .015  to  .020  inch  thick  placed  around  the  core,  and 
the  same  or  even  more  at  the  ends,  prior  to  winding  the 
wire,  will,  in  most  cases,  be  sufficient.  It  is  well  to  soak 
the  cardboard  in  paraffine  or  thin  shellac  to  keep  it  from 
absorbing  moisture.  When  there  are  many  layers,  and  par- 
ticularly when  very  fine  wire  is  used,  it  is  well  to  place 
thin  sheets  of  cardboard  between  every  'three  or  four 
layers,  particularly  under  the  outside  layer,  in  order  to  keejp 
them  smooth  and  regular.  For  the  sake  of  appearance, 
the  last  layer  should  always  be  wound  full,  which  can 
readily  be  done  when  there  are  a  large  number  of  turns, 
by  simply  omitting  or  adding  the  odd  number  of  turns, 


Field  Magnet  Coils.  169 

but  when  the  whole  number  of  windings  in  the  coil  is  com- 
paratively small  the  odd  number  of  turns  should  be 
wound  in  the  next  to  the  last  layer,  and  the  remaining 
space  filled  with  cardboard,  the  last  layer  being  then 
wound  over  all. 

When  the  coils  are  oval  it  is  well  to  wind  them  so  that 
at  the  ends  of  the  oval  the  wires  lie  in  the  position  shown  in 
figure  12,  chapter  v,  instead  of  as  shown  in  figure  13,  as  they 
may  otherwise  slip  into  the  former  position  and  thereby 
become  loose,  which  should  be  carefully  guarded  against, 
as  loose  wires,  by  the  vibrations  of  the  machine,  are  apt  to 
abrade  their  insulation. 

The  method  of  winding  shown  and  described  in  chapter 
v,  figure  14,  bringing  both  beginning  and  end  of  the 
wire  into  the  last  or  outer  layer,  may  be  used  also  in 
magnet  coils,  but  as  these  are  generally  Avound  in  lathes  it 
would  be  attended  with  such  difficulties  that  it  will,  in 
most  cases,  be  impracticable. 

In  the  ordinary  method  of  winding,  the  inner  end  of  the 
wire  which  has  to  pass  out  at  the  end  of  the  coil,  should 
be  very  wfell  insulated  with  two  or  three  extra  insulations 
where  it  passes  the  other  wires.  A  strong,  hard  cord 
wound  closely  around  this  part  of  the  wjre  and  afterwards 
shellaced  and  taped,  forms  a  very  good  insulation.  This 
end  should  be  secured  firmly  where  it  leaves  the  coil  to 
prevent  vibrations  of  the  wire.  It  is  well  to  take  one  turn 
with  this  wire  in  the  outer  layer  in  order  that  if  it  should 
break  off  short  at  the  coil  at  any  time  a  splice  can  readily 
be  made. 

When  the  wire  is  thick  and  has  been  bent  by  having 
been  wound  once  before,  it  may  readily  be  straightened  by 
drawing  it,  wfrile  it  is  being  wound,  through  a  series  of 
grooved  rollers  as  shown  in  figure  31.  The  tension  neces- 
sary in  winding  may  also  be  adjusted  by  these  rollers. 
The  practice  of  hammering  the  wire  straight  after  it  is 
wound  is  very  objectionable,  and  should  be  resorted  to  only 
when  absolutely  necessary,  and  then  only  with  great  care. 


170 


Principles  of  Dynamo-Electric  Machines. 


An  iron  hammer  should  never  he  used  for  this  except  in 
connection  with  a  piece  of  soft  wood  grooved  at  its  end  to 
fit  the  wire. 


Fig. 31 


If  it  is  desired  to  measure  the  length  of  the  wire  as  it  is 
being  wound,  it  may  be  passed  between  two  rollers  of 
known  circumference  with  a  thin  layer  of  leather  or  rub- 
ber on  the  surface,  and  an  ordinary  speed  counter  attached 
to  the  shaft  of  one  of  them,  the  length  being  calculated 
from  the  circumference  and  number  of  turns  of  this  roller. 
If  the  apparatus  in  figure  31  is  used,  the  speed  counter 
may  be  attached  directly  to  one  of  these  rollers. 

Magnet  coils  should  not  be  connected  in  multiple  arc 
unless  there  is  no  alternative,  as  there  may  be  an  unequal 
distribution  of  the  magnetism,  unless  both  the  resistance 
and  the  number  of  turns  is  the  same. 

It  is  evidently  unnecessary,  so  far  as  polarity  is  con- 
cerned, to  wind  the  coils  in  any  particular  direction,  and k 
they  may,  therefore,  always  be  wound  in  the  direction 
which  is  most  convenient,  as  they  may  always  be  so  con- 
nected with  one  another  and  with  the  binding  posts  of  the 
machine  that  the  magnets  shall  have  the  proper  po- 
larity. The  only  object  in  winding  the  coils  in  particular 
directions  is,  that  the  connections  between  the  coils  arid 
with  the  binding  posts  may  be  as  convenient  and  as  short 
as  possible  for  the  sake  of  appearance.  In  applying  Am- 
pere's laws  to  determine  the  direction  which  the  current 


Field  Magnet  Coils.  171 

must  have  in  order  to  develop  the  required  poles,  it  must 
be  remembered  to  face  the  end  of  the  coil.  The  current 
in  the  connecting  wire  between  those  ends  of  two  coils 
which  terminate  in  the  same  pole-piece,  will  describe  the 
letter  U,  while  that  between  those  ends  which  terminate 
in  the  same  yoke-piece,  will  describe  the  letter  S- 

In  making  the  preliminary  test  to  determine  the  re- 
quired ampere-turns,  it  is  well  to  examine  the  polarity  of 
the  pole-pieces  with  a  compass  needle  (being  careful  that 
the  polarity  of  the  needle  is  not  reversed  in  doing  so),  in 
order  that  the  finished  machine  may  be  connected  to  have 
the  same  polarity.  In  self-exciting  machines  it  is  also 
well  to  note  the  direction  of  rotation  and  of  the  current  of 
the  armature,  in  order  that  all  the  connections  in  the 
finished  machine  may  be  made  correctly  at  first,  as  much 
time  may  thereby  be  saved,  particularly  for  more  com- 
plicated connections,  as  in  compound  machines.  In  shunt 
or  series  machines,  it  is  evident  that  with  certain  connec- 
tions between  the  brushes  and  the  magnet  coils  the 
machine  will  give  absolutely  no  current. 

As  it  is  not  always  possible  to  carry  out  the  specifica- 
tions for  a  machine  in  all  details,  it  is  well  to  keep  a  com- 
plete record  of  all  the  parts  and  properties  of  each  ma- 
chine wherever  they  vary  from  the  original  determination 
or  plans.  Such  records  may  often  be  of  future  use,  es- 
pecially for  determining  constants  which  maybe  of  service 
in  designing  other  machines,  or  in  improving  those  already 
built. 

SERIES    MACHINES. 

For  the  purpose  of  calculating  the  coils  for  the  mag- 
nets, a  series  machine  may  be  regarded  as  a  separately 
excited  machine  whose  exciter  has  a  fixed,  definite  current 
and  a  potential  which  may  be  made  as  great  as  is  required 
for  the  coils.  Most  of  what  was  said  above  regarding 
separately  excited  machines  in  general,  and  this  class  in 
particular,  applies,  therefore,  to  series  machines  as  well. 


172  Principles  of  Dynamo-Electric  Machines. 

In  making  the  test  with  temporary  coils  for  determining 
the  required  ampere-turns,  the  magnets  should  be  excited 
until  the  armature  generates,  not  only  the  current  and 
potential  required  for  the  external  circuit,  but  in  addition 
that  which  will  be  required  by  the  magnets  of  the  finished 
machine.  This  additional  energy  can  readily  be  de- 
termined prior  to  testing  the  machine,  it  being  a  certain 
definite  proportion  of  the  whole  output,  as  described  above, 
depending  on  the  size  and  desired  efficiency  of  the 
machine.  In  a  series  machine  the  main  current  passes 
through  the  magnet  coils,  and  therefore  a  certain  amount  of 
potential  will  be  absorbed  in  the  coils  ;  in  making  the  pre- 
liminary test  for  determining  the  ampere- turns,  the  arma- 
ture should  therefore  be  made  to  generate  this  additional 
potential  by  increasing  the  excitation  of  the  temporary 
coils.  For  instance,  if  the  external  circuit  requires  1,000 
volts,  and  10  amperes,  and  if  5$  of  the  energy  be  allowed 
for  the  magnets,  the  exciting  with  temporary  magnets 
should  be  continued  until  the  potential  at  the  brushes  is 
1,000  -|-  5$  =  1,050  volts,  when  the  current  is  10  amperes. 

The  number  of  turns,  being  the  quotient  of  the  ampere- 
turns  and  the  current,  is  readily  determined.  The 
diameter  of  the  wire  is  then  calculated,  as  described  from 
the  limiting  conditions,  namely,  the  limited  coil  space,  the 
heating,  the  desired  efficiency  and  the  cost  of  the  wire.  In 
shunt  magnets  certain  values  of  the  diameter  of  the  wire 
would  make  it  impossible  to  generate  the  required  ampere- 
turns,  and  it  is  therefore  necessary  to  guard  against  this  in 
such  cases  by  careful  calculations  and  by  assuming  a  com- 
paratively large  factor  of  safety  in  the  allowable  resistance 
or  potential ;  but  in  series  machines  the  whole  current 
must  pass  through  the  magnets,  no  matter  what  the  size 
of  wire  ;  therefore  such  impossible  cases  do  not  occur,  and 
these  precautions  are  therefore  not  as  necessary,  while  the 
factor  of  safety  may  either  be  omitted  altogether  or  taken 
much  smaller  than  in  shunt  machines,  the  latter  being  pre- 
ferable as  it  is  always  more  satisfactory  to  the  constructor 


Field  Magnet  Coils.  173 

to  find  the  final  results  to  be  inside  of  the  limit  placed 
than  beyond  it.  If  for  any  reason  the  diameter  of  the 
wire  has  been  taken  smaller  than  it  should  be  it  will 
merely  decrease  the  efficiency  of  the  machine  and  increase 
the  heating,  but  it  will  not  prevent  the  machine  from 
being  used  for  generating  the  required  output  as  would  be 
the  case  in  shunt  machines. 

The  most  important  quantity  to  allow  for  in  the  factor 
of  safety  is  probably  the  self-induction  in  the  magnet 
coils,  which  acts  to  increase  their  apparent  resistance. 
It  is  a  question,  however,  whether  this  absorbs  energy  as  a 
dead  resistance  would,  or  whether  it  acts  more  like  a 
spring  or  a  cushion,  in  which  case  no  allowance  in  the 
factor  of  safety  is  necessary.  With  the  usual  solid  iron 
cores  it  is  probable  that  it  acts  like  both.  Its  value  de- 
pends on  a  number  of  quantities,  such  as  the  number  of 
windings,  the  current,  the  number  of  armature  coils,  the 
speed  of  rotation  of  the  armature  and  the  nature  of  the 
iron  cores  ;  for  ordinary  machines  5  to  10$  of  the  potential 
lost  in  the  magnet  coils  will  probably  be  a  sufficient  factor 
of  safety  to  cover  all  contingencies  in  series  machines. 

As  it  is  impossible  to  calculate  such  a  complex  machine 
as  a  dynamo  with  the  same  accuracy  as  is  attainable  in 
simpler  apparatus,  it  may  be  found  upon  testing  the 
finished  machine  that  its  output  differs  slightly  from  what 
was  required.  If  the  test  described  above  for  the  empiri- 
cal determination  of  the  required  ampere-turns  has  been 
made,  almost  all  the  errors  and  indefinite  or  indeterminate 
factors  which  enter  into  the  calculation  of  a  dynamo,  have 
thereby  been  eliminated,  as  the  chief  elements  of  error 
occur  in  the  calculation  of  the  magnetism  (which  depends, 
among  other  things,  on  the  quality  of  the  iron,  on  the  re- 
lation of  the  ampere-turns  to  the  magnetism  induced,  and 
on  the  magnetic  leakage),  also  in  the  calculation  of  the  in- 
duction in  the  armature,  in  the  self-induction,  and  in  the 
adjustment  of  the  brushes  to  the  position  of  least  spark- 
ing. These  chief  causes  of  inaccuracy  having  been 


174  Principles  of  Dynamo- Electric  Machines. 

eliminated  by  the  test  for  the  ampere-turns,  any  differences 
which  may  be  found  to  exist  after  the  machine  is  com- 
pleted, should  be  inappreciable  or  at  least  very  small. 

If  it  is  desired  to  correct  such  differences,  and  if  there  is 
no  regulator  attached  to  the  machine  by  which  this  can  be 
done,  the  speed  may  be  altered  accordingly.  To  de- 
termine this  correction  for  series  machines,  run  the  finished 
dynamo  and  adjust  the  resistance  in  the  external  circuit 
until  the  current  is  the  required  strength  after  the  machine 
has  been  running  with  full  load  for  a  number  of  hours  and 
has  attained  its  highest  temperature.  Measure  its  speed, 
s,  and  the  potential  FJ  at  the  binding  posts  when  thus 
running,  then  if  the  required  potential  is  V1,  the  speed  at 
which  it  should  be  run,  to  correct  for  this  potential,  is 

s  F1 


SHUNT   MACHINES. 

A  shunt  machine  may,  as  far  as  the  calculation  of  the 
winding  of  the  coils  is  concerned,  be  considered  as  a 
separately  excited  machine  whose  exciter  has  a  fixed 
potential,  and  the  current  from  which  may  be  made  as 
great  as  is  required  for  the  coils.  Most  of  what  was  said 
above,  regarding  separately  excited  machines  in  general, 
and  this  class  in  particular,  applies,  therefore,  also  to  shunt 
machines. 

In  making  the  test  for  ampere-turns,  with  temporary 
coils,  the  magnets  should  be  excited  until  the  armature 
generates  the  required  potential  when  the  current  is  equal 
to  that  which  is  required  in  the  external  circuit  and,  in 
addition,  that  which  is  to  be  used  in  the  coils.  What  was 
said  regarding  the  potential  absorbed  in  the  coils  of  a 
series  machine  applies,  therefore,  equally  well  to  the 
current  required  in  the  coils  of  a  shunt  machine. 

The  number  of  turns  on  the  magnets  is  not  always 
definite  as  in  a  series  machine,  but  like  the  diameter  of 


Field  Magnet  Coils.  175 

the  wire,  is  generally  limited  to  a  certain  range  of  values, 
from  which  that  which  best  meets  the  conditions  for  cheap- 
ness or  for  efficiency,  may  be  chosen,  as  described  before. 
There  is  a  definite  limit  to  decreasing  the  size  of  the  wire 
and  therefore  to  increasing  the  number  of  turns,  for  it  is 
evident  that  when  the  wire  is  smaller  than  a  certain  fixed 
size,  which  can  readily  be  determined  for  each  case,  the 
total  resistance  will  be  so  high  that  the  coils  will  not  take 
the  current  required  to  generate  the  necessary  ampere- 
turns.  It  is  necessary,  therefore,  in  shunt  machines,  to  use 
a  comparatively  large  factor  of  safety  in  order  to  guard 
carefully  against  passing  this  minimum  value  for  the 
diameter  or  maximum  value  for  the  number  of  turns. 
The  only  way  to  correct  such  an  error  in  the  finished 
machine  is  to  increase  the  speed.  The  maximum  limit  to 
the  diameter  of  the  wire  is  not  so  sharply  defined,  being  de- 
pendent on  the  heating  of  the  coils.  Should  the  coils  be 
required  to  generate  a  comparatively  large  number  of 
ampere-turns  in  a  small  coil  space,  it  may  always  be 
accomplished  at  the  expense  of  the  efficiency,  by  increas- 
ing the  diameter  of  the  wire,  thereby  increasing  the 
current  and  diminishing  the  number  of  turns,  provided 
only  that  the  heating  limit  is  not  exceeded. 

The  factor  of  safety  used  in  calculating  the  diameter 
from  the  formula  is,  as  in  series  machines,  dependent  on  a 
number  of  factors.  In  ordinary  cases  20  to  25$  will 
probably  be  sufficient,  depending  on  the  number  of  errors 
which  must  be  allowed  for. 

Should  any  slight  differences  be  found  to  exist  between 
the  actual  and  the  desired  output  of  the  finished  machine, 
they  may  be  corrected  by  a  slight  alteration  in  the  speed. 
An  increase  of  the  speed  will  evidently  increase  the  induc- 
tion in  the  armature,  that  is,  the  potential  at  the  brushes 
and  at  the  magnet  coils  ;  this  will  increase  the  current  in 
these  coils,  and,  therefore,  the  magnetism,  which  will  again 
increase  the  potential.  A  slight  increase  in  speed  will 
therefore  increase  the  potential  in  a  much  greater  propor- 


176  Principles  of  Dynamo-Electric  Machines. 

tion.  For  small  differences  it  may  be  assumed  to  vary 
approximately  as  the  square  of  the  speed.  To  make  this 
correction,  run  the  finished  machine  until  it  has  attained 
its  highest  temperature,  and  adjust  the  external  resistance 
until  the  current  has  the  proper  strength,  independently 
of  the  potential.  Measure  its  speed,  s,  and  the  potential 
"Fat  the  binding  posts  when  thus  running,  then  if  the  re- 
quired potential  is  F"1  the  speed  at  which  it  should  be  run 
to  correct  for  this  potential  is 


V 


COMPOUND  MACHINES. 

As  the  magnets  for  compound  machines  generally  con- 
sist of  both  shunt  and  series  coils,  their  winding  is 
governed  by  the  same  general  principles  as  those  given 
above.  There  remains  only  to  determine  how  much  of  the 
required  magnetism  is  to  be  generated  by  shunt  coils  and 
how  much  by  series  coils,  which  can  readily  be  determined 
from  the  functions  of  these  coils.  For  instance,  in  a 
simple  shunt  machine  the  difference  of  potential  at  the 
terminals  will  fall  with  an  increase  of  the  current,  because 
there  will  be  more  of  the  potential  absorbed  in  the 
machine  itself  in  sending  the  increased  current  through 
the  armature,  thus  making  the  useful  or  available 
difference  of  potential  so  much  less.  This  will  in  turn 
diminish  the  current  in  the  shunt  coils,  which  will,  by  de- 
creasing the  magnetism,  diminish  the  potential  still  more. 
If,  therefore,  constant  potential  is  desired  in  a  simple 
shunt  machine,  it  will  be  necessary  to  make  the  armature 
resistance  exceedingly  small  in  order  that  the  potential 
which  is  absorbed  in  it  will  be  inappreciably  small  as  com- 
pared with  the  total  electromotive  force  of  the  machine. 
Although  this  can  be  done  it  is  probable  that  the  shunt 
machine  will  thereby  become  very  large  and  heavy  as 


Field  Magnet  Coils.  177 

compared  to  the  output.  If,  however,  series  coils  of  a 
certain  number  of  turns  be  added  to  the  shunt  coils,  an  in- 
creased current  in  the  armature  by  passing  through  these 
series  coils  will  increase  the  magnetism  just  enough  to 
regenerate  the  increased  potential  absorbed  in  the  arma- 
ture by  the  greater  current,  and  therefore  the  available 
or  useful  potential  will  be  kept  constant.  The  function  of 
the  shunt  coils  in  a  compound  machine  is,  therefore,  to 
generate  the  required  potential  for  a  small  current,  while 
that  of  the  series  coils  is  to  generate  the  potential  absorbed 
in  the  machine  itself.  For  an  armature  with  a  relatively 
small  resistance  there  will  be  required  only  a  few  turns  in 
the  series  coils,  while  if  its  resistance  is  relatively  great 
the  number  of  series  turns  will  have  to  be  proportionally 
greater. 

If  the  series  coils  have  a  relatively  high  resistance,  due 
either  to  a  comparatively  large  number  of  turns  or  to  a 
limited  coil  space,  the  potential  absorbed  by  them  will  be- 
come appreciable  and  will  again  cause  the  potential  of  the 
machine  to  fall  with  an  increase  of  current.  This  can 
readily  be  corrected  by  adding  a  few  more  turns  of  the 
series  coils  to  regenerate  this  lost  potential.  When  con- 
stant potential  machines  are  used  for  incandescent  lights 
there  is  a  certain  amount  of  potential  lost  in  the  leads 
which  increases  with  the  current;  the  lamps  will  conse- 
quently grow  less  bright  as  more  of  them  are  turned  on. 
In  order  to  correct  this,  the  writer  suggested  some  years 
ago,  to  wind  such  compound  machines,  not  for  a  constant 
potential  as  was  usual,  but  for  a  potential  which  increases 
with  the  current,  in  order  that  the  available  potential  at 
the  lamps  and  not  at  the  machine  remains  constant  for  all 
currents.  This  is  done  by  simply  increasing  the  series 
windings  still  more,  in  order  to  correct  for  the  loss  of 
potential  in  the  leads  in  addition  to  that  lost  in  the  series 
coils  and  in  the  armature.  Such  machines  are  now  largely 
used  in  place  of  the  older  constant  potential  machines. 
The  chief  advantage  obtained,  apart  from  the  constant 


178 


Principles  of  Dynamo-Electric  Machines. 


brightness  of  the  lamps,  is  that  the  loss  in  the  leads  may 
then  be  made  comparatively  great,  15  to  20$,  or  for  great 
distances  even  30^,  by  which  a  great  saving  of  copper 
will  be  gained,  the  cost  of  the  additional  lost. power  in  the 
leads  being  small  as  compared  with  the  interest  on  the 
cost  of  the  copper  of  the  leads  if  this  loss  were  made 
small. 

There  are  two  methods  of  making  the  connections  of 
compound  machines,  shown  diagrammatically  in  figures  32 
and  33.  In  the.  first  of  these,  known  as  the  ordinary  com- 
pound machine,  the  shunt  coils  marked  Sh  are  connected 
to  the  brushes,  while  the  whole  external  current  flows 
through  the  series  coils  Sr,  the  binding  post  of  the  machine 
being  -j-  and  —  P.  In  the  second  method,  figure  33,  known 
as  the  "long  shunt"  compound  machine,  the  shunt  coils 
are  connected  to  the  poles  of  the  machine  instead  of  to  the 
armature  brushes,  and,  therefore,  the  whole  armature 
current  passes  through  the  series  coils.  The  differences 
in  the  two  methods  will  become  more  apparent  when  con- 
sidering the  calculations  of  the  winding.  In  large,  well 
proportioned  machines  with  few  series  windings,  the 
differences  are  very  slight  if  at  all  appreciable. 


Fig.  32 


Fig.  33 


The  test  with  temporary  coils  for  ascertaining  the  re- 
quired ampere-turns  should  be  made  in  the  same  way  as 
before,  but  instead  of  testing  the  machine  for  full  load 
only,  the  test  should  be  continued  for  a  large  number  of 
different  loads,  say  15  to  20,  varying  from  full  load  down 
to  open  circuit,  measuring  in  each  case  the  ampere-turns 


Field  Magnet  Coils. 


179 


which  are  required  to  excite  the  machine  to  the  desired 
potential  at  the  armature.  Before  making  this  test  the 
machine  should  be  examined  for  magnetic  saturation,  as 
will  be  described  in  a  subsequent  chapter,  in  order  to 
ascertain  whether  the  iron  of  the  frame  is  properly  pro- 
portioned for  compound  machines  and  to  find  what  the 
maximum  limit  to  the  ampere-turns  is.  It  is  very 
necessary  in  compound  machines  not  to  have  them  over- 
saturated  and  to  have  the  useful  magnetism  increase  as 
nearly  proportional  to  the  magnetizing  current  as  possible, 
which  is  most  nearly  accomplished  by  having  much  iron 
of  good  quality,  evenly  distributed  and  not  too*  near 
saturation,  otherwise  there  will  be  only  a  rough  approxi- 
mation to  self -regulation. 

The  proper  position  of  the  brushes  should  be  ascertained 
and  fixed  once  for  all ;  it  should  not  be  required  to  be 
altered  for  different  loads,  for  it  is  evident  that  the 
machine  is  not  self-regulating  if  the  brushes  require  to  be 


AMPERE-TURNS 


adjusted  for  different  loads.  A  well  proportioned  com- 
pound machine  should  not  spark  perceptibly  for  the  same 
position  of  the  brushes,  even  when  the  full  load  is  thrown 
off  and  on  suddenly. 

Having  made  all  these  tests  and  adjustments,  the  com- 


180  Principles  of  Dynamo-Electric  Machines. 

pounding  of  the  coils  may  be  calculated  according  to  the 
following  method,  which  will  be  limited  to  machines 
compounded  for  constant  potential  at  the  machine  or  at  the 
lamps,  as  these  are  the  only  kinds  that  have  been  found 
practicable.  On  a  piece  of  cross  sectional  paper  lay  off 
along  the  vertical  line,  O  Y,  figure  34,  on  any  convenient 
scale,  the  currents  which  the  machine  generated  in  the 
above  described  test,  and  along  horizontal  lines  through 
these  points  lay  off  the  ampere-turns  required  in  each  case 
to  excite  the  machine  for  constant  potential,  and  draw  a 
line,  a  b,  through  the  points  thus  located.  This  line  will, 
even  in  the  best  machines,  be  somewhat  curved,  particularly 
at  the  ends.  The  magnetization  of  the  machine  should  not 
be  continued  beyond  the  point  marked  b,  where  the  line 
begins  to  have  a  decided  curvature,  as  the  compounding  is 
possible  only  for  the  tolerably  straight  portion  of  the 
curve.  Draw  a  straight  line,  c  d,  through  the  most  im- 
portant points  of  the  line,  a  b,  that  is  through  such  points 
at  which  the  machine  is  to  be  used  most  frequently  ;  this 
line  will  then  represent  the  nearest  approach  to  constant 
potential  for  that  machine  ;  the  greater  the  variation  of 
the  straight  from  the  curved  line  the  greater  will  be  the 
variation  of  the  potential  from  a  constant.  The  deviation 
from  this  straight  line  will  therefore  show  at  once  whether 
the  machine  is  suitably  proportioned  for  being  wound  for 
constant  potential. 

This  line  a  b  represents  a  certain  proportion  of  the  series 
and  shunt  coils.  The  horizontal  distances  between  it  and 
the  line  O  I7" represent  the  ampere- turns  required  for  the  re- 
spective currents  in  the  external  circuit.  These  distances 
are  made  up  of  a  constant  portion,  ef,  and  a  gradually  in- 
creasing portion,/^,  which  is  greater  in  proportion  to  the 
external  current.  As  the  shunt  coils  are  to  be  connected 
to  the  poles  of  the  machine  or  to  the  armature  where  the 
potential  is  to  be  constant,  the  current  in  them  and,  there- 
fore, the  number  of  ampere-turns  is  constant  for  all  loads. 
The  constant  portion  e  f  or  o  c  of  the  whole  number  of 


Field  Magnet  Coils.  181 

arapere-turns  should  therefore  be  allotted  to  the  shunt 
coils,  and  as  the  ampere-turns  of  the  series  coils  increases 
in  proportion  to  the  current  in  the  external  circuit,  that 
portion  of  the  whole  number  of  ampere-turns  which  lies 
between  f  c  and  d  c  should  be  generated  by  the  series 
coils.  The  shunt  coils  are  therefore  calculated  as  for  an 
ordinary  shunt  machine  requiring  the  ampere-turns  rep- 
resented by  the  distance  eforo  c,  while  the  series  coils 
are  calculated  as  for  an  ordinary  series  machine  requiring 
the  ampere-turns  represented  by  /  g  when  the  current  is 
that  represented  by  o  e. 

The  total  energy  allotted  to  the  magnets,  should  be  di- 
vided between  the  series  and  the  shunt  coils  in  proportion 
to  the  ampere-turns  generated  by  them  respectively,  at  full 
load,  the  series  coils  absorbing  potential,  and  the  shunt 
coils  current.  In  determining  the  coil  space  for  the  shunt 
coils,  care  should  be  taken  to  correct  for  that  occupied  by 
the  series  coils  which  are  usually  wound  first,  as  the  real 
resistance  might  otherwise  be  considerably  higher  than 
the  calculated,  owing  to  the  increased  length.  In  deter- 
mining the  heating  limit  to  the  diameter,  the  total  maxi- 
mum number  of  watts  which  may  be  dissipated  in  the  coils 
may  be  calculated  from  one  of  the  formulae  given  above, 
the  cooling  surface  being  that  of  the  outside  coil  only  ; 
this  total  may  then  be  divided  between  the  coils  in  pro- 
portion to  their  number  of  ampere-turns,  or  in  any  other 
proportion  provided  only  that  the  sum  of  the  watts  lost  in 
both  does  not  exceed  this  total. 

The  general  method  just  described,  applies  only  to  large, 
well  proportioned  machines,  or  to  others  when  only  a 
rough  approximation  is  desired  ;  for  smaller  machines,  or 
when  the  machine  is  to  correct  for  loss  of  potential  in  the 
leads  for  incandescent  lamps,  or  in  cases  where  accuracy 
is  desired,  certain  corrections  are  required  in  the  calcula- 
tion of  the  compounding  of  the  coils.  These  corrections 
will  be  different,  according  to  which  of  the  two  connections 
shown  in  figures  32  and  33  are  used  ;  they  may  be  briefly 


182 


Principles  of  Dynamo-Electric  Machines. 


described  by  the  following  diagrams,  their  importance  in 
different  cases  being  rendered  apparent  by  their  actual  val- 
ues. Referring  first  to  figure  32,  it  is  evident  that  if  the 
shunt  current  is  appreciably  great,  it  should  be  subtracted 
from  the  armature  current  to  give  the  real  external  and 
series  winding  current.  In  figure  35,  therefore,  in  which 
c  d  represents  the  same  as  'c  d  in  figure  34,  this  shunt  cur- 
rent should  be  laid  off  at  o  h  /  the  external  or  series  winding 
current  should,  therefore,  be  measured  from  A,  instead  of 
from  o,  and  the  shunt  winding  calculated  for  h  i  ampere- 
turns  instead  of  oc.  Furthermore,  if  the  potential  is  con- 
stant at  -[-  and  —  jP,  figure  32,  it  will  have  to  increase 
slightly  at  the  terminals  of  the  shunt  coil  for  full  load, 
owing  to  that  consumed  in  the  series  coils,  which  will  be 
appreciably  great  when  the  armature  resistance  is  rela- 
tively great ;  the  magnetization  of  the  shunt  coils  will, 
therefore,  increase  slightly  with  an  increased  load,  instead 
of  being  constant.  This  increased  potential  for  the  great- 


e- 


fo   dlli 


h 


Fig.  35 


Fig.  36 


est  load  can  readily  be  calculated  by  Ohm's  law  from 'the 
resistance  and  current  in  the  series  coils  ;  let  this  be  v, 
and  let  Vbe  the  potential  at  the  poles  -f-  and  —  P,  then  that 
at  the  shunt  coils,  for  full  load  will  be  V  +  v  ;  if  h  e  is  the 
maximum  external  current  lay  off  e  k,  so  that  it  bears  the 


Field  Magnet  Coils.  183 

same  proportion  to  h  i,  as  "P-j-  v  does  to  "F;  in  other  words, 

V  4-  v 

make  e  k  =  — ^7—  (h  i),  then  e  k  will  represent  the  mag- 
netism of  the  shunt  coils  at  full  load,  and  will  be  slightly 
greater  than  ef,  figure  34.  The  potential  v,  lost  in  the 
series  coils  must  be  generated  again  by  a  few  additional 
windings  of  the  series  coils  ;  if  it  takes  e  d  ampere-turns  to 
generate  V volts,  it  will  require  el  ampere-turns  to  gener- 
ate V  -\-  v  volts,  in  which  by  simple  proportion,  e  I  == 

F+  v 

'  (ed).  The  line  H,  therefore,  represents  the  cor- 
rection of  the  line  d  i,  and  the  series  coils  should  be  cal- 
culated from  the  ampere-turns  represented  by  Jc  I,  and  the 
external  current,  he,  instead  of  from  fg  and  o  6, 
respectively,  in  figure  34. 

If  the  connections  are  as  in  figure  33,  these  corrections 
will  be  slightly  different.  If  the  potential  is  constant  at 
-j-  and  —  P9  the  current  and  magnetization  of  the  shunt 
coils  is  constant,  and  no  correction  is  necessary  for  them. 
In  figure  36,  therefore,  lay  off  as  before,  o  h,  equal  to  the 
shunt  current,  then  h  m  =  oc,  will  be  the  shunt  ampere- 
turns.  To  correct  the  series  coils  for  the  potential  lost  in 
them,  lay  off  e  I,  determined  as  in  figure  35,  then  Ic,  will 
be  the  correction  of  the  line  d  c,  and  the  series  coils 
should,  as  before,  be  calculated  from  the  ampere-turns,  k  I, 
and  the  current  e  h.  In  both  figures  35  and  36,  these  cor- 
rections are  greatly  exaggerated  for  the  sake  of  clearness. 
As  the- shunt  current  in  figure  33  must  also  pass  through 
the  series  coils,  they  act  to  a  certain  extent  as  shunt  coils 
also,  though  they  should  not  be  included  in  the  calculation 
of  the  latter.  In  figure  36,  m  i  represents  the  ampere- 
turns  which  are  generated  in  these  series  coils  by  the  shunt 
coil  current. 

If  the  machine  is  furthermore  to  correct  for  the  fall  of 
potential  in  the  leads,  the  simplest  way  to  determine  the 


184  Principles  of  Dynamo-Electric  Machines. 

winding,  is  to  make  the  test  for  ampere-turns  by  passing 
the  main  current  through  a  resistance  equal  to  that  which 
the  leads  are  to  have,  and  thence  through  the  lamps  or 
other  resistance,  and  to  excite  the  machine  so  that  the  po- 
tential at  the  end  of  this  lead  resistance  remains  the  same 
for  different  loads.  The  calculations  are  then  made  as  be- 
fore, except  that  the  potential  at  the  shunt  coils  is  not  that 
measured  at  the  ends  of  these  leads,  but  that  at  the  ma- 
chine, which  is,  of  course,  higher,  and  may  be  either 
measured  during  the  test  or  calculated  from  the  resistance 
of  the  leads  and  the  current.  If  this  test  cannot  con- 
veniently be  made  with  a  resistance  equal  to  that  of  the 
leads,  the  correction  of  the  ampere-turns  may  be  calcula- 
ted in  the  same  way  as  described  above  for  correcting  for 
the  loss  in  the  series  coils,  except  that  the  shunt  coils  are 
not  determined  from  the  potential  at  the  ends  of  these 
leads,  but  only  from  that  at  the  machine. 

The  relative  values  of  the  two  methods  of  connecting, 
shown  in  figures  32  and  33,  may  be  s^en  by  comparing 
figures  35  and  36.  When  the  corrections,  shown  exaggera- 
ted here,  are  small,  the  two  methods  are  practically  tne 
same,  which  will  be  the  case  for  large,  well  proportioned 
machines.  In  small  machines  these  corrections  become 
more  important,  in  which  case  there  will  probably  be  a 
slight  advantage  in  favor  of  the  second,  as  the  shunt  coils, 
which  are  the  more  expensive,  are  somewhat  smaller,  as 
seen  from  the  distance,  h  m,  figure  36,  compared  to  h  i,  fig- 
ure 35,  besides  being  subjected  to  a  lower  potential ;  the 
series  coils  are  consequently  somewhat  larger,  but  the 
whole  coils  will  be  less  bulky  than  in  the  first  method. 

Figure  34,  as  well  .as  the  corrections  shown  in  35  and  36, 
indicate  what  the  most  desirable  general  proportions  are 
for  compound  machines.  The  curved  line,  a  b,  should  be 
as  straight  as  possible,  which  may  be  accomplished  by 
using  a  good  quality  of  soft  iron  in  the  field,  by  making 
the  cross-section  large  enough  to  be  well  below  the  point 
of  saturation,  and  by  distributing  it  so  that  all  parts  are 


Field  Magnet  Coils.  185 

magnetized  to  about  the  same  degree,  instead  of  having 
some  parts  over-saturated  while  others  are  far  below  satu- 
ration, as  is  often  the  case.  Furthermore,  the  inclination 
of  the  line  c  d,  to  the  vertical  should  be  as  small  as  possi- 
ble, as  the  variations  of  the  potential  which  the  series 
coils  have  to  correct  will  then  be  less.  This  is  accom- 
plished by  making  the  armature  resistance  as  small  as 
practicable.  To  avoid  shifting  of  the  brushes,  the  number 
of  armature  windings,  and  particularly  the  number  of 
windings  per  armature  coil,  should  be  as  small  as  possible. 
In  general,  all  that  was  said  in  previous  chapters  regarding 
the  most  desirable  proportions  of  armature  and  field,  ap- 
plies particularly  to  compound  machines,  as  it  is  of  the 
greatest  importance  to  have  these  well  proportioned.  The 
magnets  should  respond  quickly  and  readily  to  changes  of 
magnetization,  in  order  that  regulation  may  be  easily 
effected  by  the  ampere-turns,  over-saturation  or  even  too 
close  approximation  to  saturation  should,  therefore,  be 
carefully  guarded  against. 

From  the  principles  of  constant  potential  compound  ma- 
chines, it  is  evident  that  they  will  be  self -regulating  only 
for  variations  in  the  current,  but  not  for  variations  in  the 
speed  or  in  the  resistance  of  the  coils  due  to  heating, 
neither  of  these  can  be  compensated  for  by  compound 
winding,  and  it  is,  therefore,  necessary  to  maintain  both 
the  speed  and  the  heating  as  nearly  constant  as  possible. 
Furthermore,  it  is  necessary  to  run  the  finished  machine 
at  the  same  speed  as  that  in  the  test,  as  the  proportions  of 
the  series  and  shunt  coils  will  be  quite  different  for  differ- 
ent speeds.  The  calculations  should  be  made  as  carefully 
as  possible,  as  it  is  not  possible  to  correct  any  mistake  by 
changing  the  speed  as  in  simple  shunt  or  series  machines. 
If,  however,  a  final  correction  is  found  necessary,  the  speed 
may  be  increased  and  an  adjustable  resistance  placed  in 
series  with  the  shunt  coils,  and  one  as  shunt  to  the  series 
coils  ;  these  can  be  adjusted  by  trial  to  compensate  for  the 
error,  and  should  then  remain  unaltered  for  varying  loads; 


186  Principles  of  Dynamo-Electric  Machines. 

from  the  functions  of  the  shunt  and  the  series  coils,  it  can 
readily  be  seen  which  of  these  two  should  be  adjusted.1 

1.  For  further  information  regarding  the  winding  of  field  magnet  coils,  the 
reader  is  referred  to  the  author's  "  Practical  Directions  for  Winding  Magnets  for 
Dynamos.1' 


CHAPTER   IX. 

Regulation  of  Machines. 

THE  simplest  and  often  the  most  convenient  way  of  reg- 
ulating or  adjusting  the  current  or  potential  which  is  gen- 
erated by  a  machine,  is  to  move  the  brushes  toward  or  from 
the  neutral  line.  It  has  been  shown  in  chapter  iv  that 
the  halves  of  the  continuous  winding  of  an  armature  are 
connected  in  multiple  arc  with  each  other,  and  that  the  to- 
tal electromotive  force  generated  in  each  half  is  equal  to 
the  sum  of  the  electromotive  forces  of  the  separate  coils  in 
that  half;  if,  therefore,  the  brushes  are  moved  away  from 
the  neutral  line,  say  a  distance  equal  to  the  width  of  one 
commutator  bar,  it  is  evident  that  one  coil  on  each  side  of 
the  armature  will  thereby  have  been  switched  from  one 
side  to  the  other  of  the  line  joining  the  brushes  (see 
figures  6  or  8,  chapters  iv.  and  v) .  As  the  direction  of 
the  induced  current  will  tend  to  remain  the  same  in  these 
two  coils,  it  will  evidently  be  opposed  to  the  other  coils  as 
now  connected,  and  will  therefore  dimmish  the  available 
electromotive  force  at  the  two  brushes.  *For  instance,  if 
there  were  64  armature  coils  and  if  each  coil  generated 
three  volts,  there  would  be  32  coils  in  series  in^each  half, 
giving  3  X  32=96  volts  ;  if  now  the  brush  be  moved  the 
width  of  one  commutator  bar,  there  would  be  31  coils  left, 
generating  93  volts,  but  these  would  be  opposed  by  the 
three  volts  of  the  coil  which  was  added,  thus  leaving  90 
volts.  This  will  continue  to  decrease  by  continuing  the 
displacement  of  the  brushes  until  they  have  been  moved 
through  90°  when  the  coils  will  all  oppose  and  neutralize 
each  other.  In  actual  practice  the  results  would  be  modi- 
fied somewhat,  quantitatively,  as  the  electromotive  force 

(187) 


188  Principles  of  Dynamo-Electric  Machines. 

induced  in  the  coils  near  the  neutral  line  is  generally  less 
than  in  the  others,  thus  making  the  regulation  more  gradual 
at  first  ;  furthermore,  the  direction  of  magnetization  of  the 
armature  by  its  own  current,  will,  of  course,  follow  the 
movement  of  the  brushes,  and  therefore  will  tend  to  move 
the  neutral  line  in  the  same  direction;  but  in  well  built  ar- 
matures with  few  windings,  this  effect  will  be  small  if  at  all 
appreciable,  as  the  magnetization  due  to  its  own  current 
will  be  small  as  compared  to  that  due  to  the  field. 

This  method  of  regulation  is  evidently  equally  well  ap- 
plicable for  adjusting  the  current  in  the  external  circuit, 
as  well  as  the  electromotive  force,  for  the  former  may  al- 
ways be  regulated  by  proper  adjustment  of  its  electromo- 
tive force.  It  is  furthermore  evident  that  by  thus  dimin- 
ishing the  electromotive  force  or  current,  or  both,  a  nearly 
proportionate  amount  of  power  will  be  saved,  as  the  oppo- 
sing electromotive  force  does  not  generate  an  opposing  cur- 
rent, and  therefore  no  energy  is  required  by  it.  This  can 
be  shown  by  moving  the  brushes  through  about  90°,  in 
which  case  no  current  will  be  generated  and  the  dynamo 
will  run  light,  that  is,  without  consuming  power. 

An  objection  to  this  form  or  regulation,  and  one  which 
makes  it  prohibitive  in  most  cases,  is  that  it  generally  causes 
very  bad  sparking  when  the  brushes  are  too  far  from  the 
neutral  line.  It  is  well  known  to  all  who  have  adjusted 
the  brushes  of  a  dynamo,  that  there  is  one  definite  posi- 
tion, and  generally  only  one,  in  which  there  is  least  spark- 
ing, and  that  the  sparking  is  increased  by  altering  this  po- 
sition. This  is  partially  due  to  the  fact  that  at  least  one 
coil  is  continually  short  circuited  by  each  brush,  and  when 
the  brushes  are  moved  too  far  from  the  neutral  line  this 
coil,  instead  of  being  "  dead,"  that  is,  without  an  induced 
electromotive  force  in  it,  is  "  alive,"  that  is,  has  electro- 
motive force  induced  in  it,  and  therefore  on  being  short  cir- 
cuited, it  will  have  a  more  or  less  great  local  current  cir- 
culating in  it,  which,  even  if  the  electromotive  force  itj 
small,  may  be  very  great,  as  the  resistance  is  small  ;  when 


Regulation  of  Machines.  189 

this  current  is  broken  at  the  end  of  the  brush,  it  evidently 
will  spark  more  or  less  badly. 

This  method  of  regulation,  when  not  accompanied  by 
any  regulation  of  the  field,  is  therefore  not  to  be  recom- 
mended, except  for  small  machines  in  which  the  sparking 
is  not  objectionable,  or  for  machines  with  many  armature 
windings  and  weak  field  in  which  the  neutral  line  will  shift 
with  the  brush  line,  or  in  such  inferior  machines  in  which 
the  sparking  is  very  bad  in  all  position  of  the  brushes,  or 
in  such  machines  as  the  Thomson-Houston  in  which  the 
evil  effects  of  the  sparking  are  avoided  to  a  great  extent 
by  a  blast  of  air  between  the  commutator  bars. 

It  may  be  remarked  here  that  the  position  of  the  brushes 
which  gives  least  sparking,  is  not  always  coincident  with 
the  position  for  greatest  electromotive  force.  It  is  evident 
that  they  should  always  be  adjusted  to  the  former  and  not 
to  the  latter,  which  may  always  be  accomplished  by  de- 
termining the  winding  of  the  magnets  as  described  in  the  last 
chapter. 

The  second  method  of  regulating  the  electromotive 
force,  and  therefore  the  current,  is  to  vary  the  speed  of 
the  dynamo,  but  as  this  is  in  most  cases  impracticable,  it 
need  not  be  discussed  here.  What  was  said  in  previous 
chapters  concerning  the  effect  of  the  speed  will  enable  one 
to  determine  the  amount  of  the  adjustment;  for  instance, 
the  effect  of  a  certain  change  of  speed  is  much  greater  for 
a  shunt  than  for  a  series  machine,  under  certain  conditions 
in  the  external  circuit.  A  change  of  speed  may  in  many 
cases  be  accompanied  by  a  change  of  position  of  the  line 
of  least  sparking,  and  should  in  those  cases  be  accom- 
panied by  a  change  of  position  of  the  brushes. 

A  third  method  of  regulation  is  to  use  a  variable  dead 
resistance  in  the  external  circuit,  which  will  absorb  a  cer- 
tain amount  of  energy,  thus  enabling  the  rest  of  the  energy, 
that  is  the  useful  part,  to  be  adjusted.  As  this  method 
wastes  energy,  its  general  use  is  not  to  be  recommended  ; 
but  in  certain  special  cases,  belonging  more  particularly  to 


190  Principles  of  Dynamo- Electric  Machines. 

the  subject  of  systems  of  distribution  rather  than  dynamos, 
there  are  advantages  gained  by  this  method  which  cannot 
be  attained  as  readily  by  other  methods,  or  in  other  words, 
the  advantages  gained  are  commensurate  with  the  cost  of 
the  wasted  power.  To  calculate  the  most  economical  form 
of  such  resistances,  proceed  as  follows:  determine  by  cal- 
culation, or  preferably  by  trial  under  actual  working  con- 
ditions, what  the  difference  of  potential,  V,  is,  which  must 
be  absorbed  by  each  of  the  successive  adjustments  of  the 
resistance;  also  the  current  G,  which  flows  through  the 

resistance  in  each  case;  the  quotients  —  will    be   the    resis- 

C 

ances.  If  german-silver  wire  or  flat  bands  are  to  be  used, 
their  cross-section  is  next  determined  from  the  heating 
limit.  If  convenient  and  reliable  formulas  are  at  hand 
this  may  be  calculated  directly,  the  necessary  length  being 
then  readily  determined  from  this  cross-section,  the  resis- 

F  ' 

tances  — ,  and  the  specific  resistance  of  german  silver.    But 
O 

in  the  absence  of  such  formulas,  or  if  the  specific  resistance 
of  the  german  silver  (which  varies  greatly)  is  not  known, 
the  following  experimental  determination  will  be  found  re- 
liable and  simple:  Take  as  long  a  piece  as  convenient  of 
the  wire  or  the  bands  of  which  the  resistances  are  to  be 
made,  measure  its  length,  and  mount  it  as  an  open  spiral  or 
otherwise,  as  it  is  to  be  mounted  in  the  finished  resistance, 
^ass  a  current  through  it  from  a  dynamo  or  battery,  and 
increase  this  current  slowly  until  the  wire  has  been  raised 
to  as  high  a  temperature  as  is  permissible,  allowing  suffi- 
cient time  for  the  temperature  to  attain  its  greatest  value; 
then  measure  the  current  c,  passing  through  it,  and  the 
difference  of  potential  v,  at  the  terminals  of  the  wire. 
The  length  of  the  wire  (of  the  same  cross-section)  for  the 
required  resistance  which  is  to  absorb  V  volts,  will  then 
evidently  be,  Fdivided  by  v,  and  multiplied  by  the  length 
of  this  trial  piece;  the  number  of  such  wires  which  must 


Regulation  of  Machines.  191 

be  placed  in  multiple  arc  in  the  resistance  for  the  current 
C,  is  evidently  C  divided  by  c.  . 

By  mounting  this  trial  wire  in  different  ways,  and  raising 
it  in  each  case  to  the  same  temperature,  the  most  econom- 
ical method  of  mounting  may  readily  be  determined;  the 
object  is,  of  course,  to  present  as  much  surface  to  the  free 
access  of  air,  as  possible.  Up  to  a  certain  limit,  differing 
under  different  conditions,  it  will  be  found  to  be  more 
economical  to  use  a  larger  number  of  small  wires  in  mul- 
tiple arc,  rather  than  a  smaller  number  of  larger  wires. 

This  method  of  experimentally  determining  the  resist- 
ance wires,  applies  only  to  those  cases  in  which  the  wires 
are  all  of  the  same  size  as  the  test  wire,  their  number  in 
multiple  arc  being  then  made  proportional  to  the  current. 
If  different  sizes  of  wire  or  bands  are  to  be  used,  they  must 
be  calculated  by  means  of  the  well-known  heating  formulae 
for  wires.  In  many  cases  the  following  modification  of 
the  formula  will  be  found  convenient  for  round  wires: 
Make  the  same  test  as  before,  with  any  convenient 
wire,  measuring  merely  its  diameter,  say  in  mils,  and  the 
maximum  current.  Cube  this  diameter,  and  divide  it  by 
the  square  of  the  current;  this  will  give  a  number  or  con- 
stant, which  can  be  used  to  calculate  the  diameter  of  any 
other  wire  of  the  same  material,  but  for  a  different  current 
which  will  not  heat  it  to  a  higher  temperature  than  that  of 
the  test  wire.  For  instance,  if  (7,  be  any  other  current, 
square  it  and  multiply  it  by  this  constant,  the  cube  root  of 
this  will  then  be  the  diameter  in  mils,  which  the  wire  must 
have,  in  order  to  carry  this  current  and  not  to  heat  any 
more  than  the  test  wire.  The  required  length  of  the  wire 
is  then  determined  from  this  cross-section  and  the  resistance 
which  it  must  have. 

If  bands  of  different  width,  but  of  the  same  thickness 
are  used  in  place  of  wires,  the  width  should  be  propor- 
tional to  the  current.  If  they  are  of  the  same  width  but 
of  different  thicknesses,  the  thickness  should  be  propor- 
tional to  the  square  of  the  current.  This  shows  that  it  is 


192  Principles  of  Dynamo- Electric  Machines. 

much  more  economical  to  use  bands  as  thin  and  as  wide  as 
possible. 

The  fourth  and  most  common  method  of  regulating  the 
electromotive  force  or  current  of  a  dynamo,  is  to  vary  the 
strength  of  the  magnetizing  current  or  the  ampere-turns 
of  the  field  magnets.  The  method  of  effecting  such  ad- 
justments of  the  magnetizing  current  will  evidently  be  dif- 
ferent according  as  the  machine  is  shunt,  series,  or  separate- 
ly excited,  and  also  whether  the  machine  is  to  be  regu- 
lated for  constant  potential  or  constant  current.  In  all 
these  cases,  the  electromotive  force  of  the  machine,  but 
not  necessarily  its  current,  will  be  nearly  proportional  to 
the  magnetizing  current,  provided  the  magnets  are  not  over- 
saturated;  in  the  latter  case  the  strength  of  the  magnetiz- 
ing current  must  be  varied  through  a  greater  range,  to 
produce  the  same  change  in  electromotive  force.  The  cur- 
rent in  the  external  circuit  will,  of  course,  depend  on  the 
resistance  of  the  circuit,  and  on  the  electromotive  force, 
and  must  therefore  be  adjusted  for  different  resistances  by 
varying  the  electromotive  force. 

Separately  excited  machines  are  regulated  by  adjusting 
the  current  strength  of  the  exciter.  This  can  be  done  by 
a  variable  resistance  in  this  exciting  circuit,  or  better,  by 
regulating  the  field  current  of  the  exciter  itself  by  any  con- 
venient method.  The  former  does  not  waste  much  power 
as  it  consumes  only  a  fraction  of  the  energy  for  the  field 
current,  which  latter  is  itself  only  a  small  fraction  of  the 
total  power. 

Shunt  machines  are  regulated  by  an  adjustable  resistance 
in  the  magnet  circuit.  If  the  machine  is  to  be  regulated 
for  constant  potential,  as  when  used  for  incandescent  lamps 
in  parallel,  the  amount  of  this  regulation  is  dependent  on 
the  resistance  of  the  armature,  and  will  therefore  be  small 
for  low  resistance  armatures.  The  amount  of  this  resistance 
is  most  readily  determined  by  an  actual  trial,  that  is,  by 
running  the  machine  first  with  full  load  and  then  with  no 
load,  and  adjusting  in  each  case  any  convenient  resistance 


Regulation  of  Machines.  193 

in  the  magnet  circuit  until  the  difference  of  potential  at 
the  poles  of  the  machine  is  normal.  Measure  these  two  re- 
sistances, as  well  as  the  current  which  flows  through  them. 
The  smallest  allowable  cross-section  for  the  resistance  wire 
is  then  determined  from  this  current,  as  described  above, 
and  its  total  length  is  then  calculated  from  this  cross-sec- 
tion and  the  total  resistance.  The  wire  is  then  mounted  so 
that  successive  parts  of  it  may  be  switched  into  the  mag- 
net circuit,  the  number  of  such  parts  (usually  from  10  to  20) 
depending  on  the  desired  nicety  of  the  regulation.  As 
the  variation  in  the  current  through  this  resistance  is  com- 
paratively small,  one  size  of  wire  will  usually  suffice  for  the 
whole  resistance;  if,  however,  the  current  varies  consider- 
ably, it  will  be  more  economical  to  use  different  sizes  of 
wire  for  different  parts;  they  may  be  calculated  from  the 
heating  formula  given  above.  Such  machines  are  usually 
wound  as  described  in  the  last  chapter,  so  as  to  enable 
some  resistance  to  be  placed  in  the  magnet  circuit  even  at 
full  load,  for  the  purpose  of  adjusting  for  slight  irregular- 
ities, such  as  the  heating  of  the  magnet  coils,  lowering  of 
the  speed  of  the  engine,  etc.  The  variation  required  be- 
tween full  and  no  load  is  of  course  due  to  the  difference 
between  the  electromotive  force  and  the  difference  of  po- 
tential, as  described  before,  and  therefore  depends  on  the 
armature  resistance.  The  energy  wasted  in  the  resistance 
is  insignificantly  small. 

If  the  shunt  machine  is  to  be  regulated  for  constant  cur- 
rent with  consequent  great  variations  in  electromotive 
force,  as  for  instance  in  an  arc  light  machine  for  a  varying 
number  of  lamps,  the  determination  of  the  required  resist- 
ance is  best  done  experimentally  as  described  above,  only 
that  it  must  be  determined  for  a  large  range  of  values,  and 
not  merely  for  the  two  extremes.  The  reason  for  this,  is 
that  this  resistance  does  not  vary  in  proportion  to  the  elec- 
tromotive force,  as  the  latter  will  itself  vary  the  current  in 
the  shunt  magnet  circuit.  It  may  even  be  found  that  the 
resistance  must  be  varied  first  in  one  direction  and  then  in 


194  Principles  of  Dynamo- Electric  Machines. 

the  other,  for  regular  diminutions  of  the  electromotive 
force.  The  current  in  the  magnet  circuit  should  also  be 
measured,  and  if  it  varies  considerably,  the  resistance  wires 
may  be  made  of  different  sizes  for  the  sake  of  economy. 
The  successive  steps  of  the  adjustable  resistance  may  have 
relatively  different  values  to  effect  a  regular  increase  of 
electromotive  force.  As  shunt  machines  for  such  high  po- 
tential as  would  be  required  for  many  arc  lights  in  series 
would  necessitate  the  use  of  a  very  large  amount  of  quite 
fine  wire  for  their  magnet  coils,  they  become  very  expen- 
sive and  are  therefore  not  used  very  much.  They  possess 
no  particular  advantages  over  the  ordinary  series  machines 
if  the  latter  are,  as  is  usually  the  case,  automatically 
regulated,  and  therefore  not  subject  to  the  destructive  ef- 
fects of  the  unavoidable  short  circuits  in  the  external 
circuit. 

Shunt  machines  may  also  be  regulated  by  winding  the 
magnet  coils  in  independent  sections;  by  means  of  a  switch- 
board to  which  the  terminals  are  attached,  these  sections 
may  be  successively  cut  out  or  switched  into  the  circuit, 
thus  varying  the  ampere-turns  of  the  magnets.  But  as  this 
complicates  the  construction  greatly  and  has  no  particular 
advantage  over  the  other  method,  it  is  not  practiced  to  any 
great  extent. 

Series  machines  for  constant  current  may  be  regulated 
by  winding  the  magnet  coils  in  sections  as  just  described, 
but  the  same  objections  apply  here  as  well.  A  modification 
of  this,  is  to  solder  smaller  branch  wires  to  different  parts 
of  the  coils  and  by  means  of  a  suitable  switch-board,  to 
which  these  wires  are  led,  short  circuit  the  successive  sec- 
tions. But  though  this  is  an  improvement  of  the  method, 
it  possesses  in  general  no  particular  advantages  over  the 
next  method  to  be  described.  In  some  particular  cases  the 
advantages  may  be  sufficiently  important  to  justify  its 
preference  over  other  methods.  In  such  cases,  care  should 
be  taken  to  cut  out  the  successive  sections  so  as  not  to 
unbalance  the  field, 


Regulation  of  Machines,  195 

The  simplest  and  most  usual  method  of  regulating  series 
machines  is  to  use  an  adjustable  resistance  as  a  shunt  to 
the  magnet  circuit,  that  is,  to  connect  an  adjustable  re- 
sistance between  where  the  main  circuit  enters  and  where 
it  leaves  the  magnets.  The  successive  steps  of  such  a  re- 
sistance may  be  best  determined  by  an  actual  trial  with  any 
convenient  resistances,  which  are  afterwards  measured  and 
serve  to  calculate  the  length  of  the  resistance  wire.  If  the 
magnets  are  not  over-saturated  the  resistances  may  be  cal- 
culated (knowing  the  resistance  of  the  magnet  coils),  so  as 
to  shunt  off  such  portions  of  the  main  current  as  will  leave 
the  magnet  current  proportional  to  the  electromotive  forces 
to  be  generated.  But.  owing  to  the  fact  that  the  unknown 
self-induction  of  the  magnet  coils  acts  to  increase  their  re- 
sistance, and  also  to  the  fact  that  the  brushes  will  in  many 
cases  be  required  to  be  adjusted  together  with  the  field, 
owing  to  the  shifting  of  the  neutral  line,  such  calculations 
will  not  be  very  reliable,  and  it  is  therefore  best  to  elimi- 
nate all  such  inaccuracies  by  an  experimental  determina- 
tion. In  this  test  the  current  which  is  shunted  around  the 
magnets  and  through  the  resistance,  should  be  measured 
for  each  successive  step,  and  from  this  the  cross-section  of 
the  resistance  wire  is  determined  by  the  heating  limit,  as 
described  above.  This  current  will  vary  from  almost 
nothing  for  the  maximum  resistance,  to  almost  the  whole 
main  current  for  the  smallest  resistance,  thus  requiring 
quite  different  sizes  of  resistance  wires  for  different  steps 
of  the  regulation.  Machines  regulated  in  this  way  may 
readily  be  adjusted  to  maintain  a  constant  current  even  for 
a  short  circuit.  The  energy  wasted  in  this  resistance  being 
a  fraction  of  that  used  for  the  magnets  is  not  significant. 

Series  machines  might  also  be  regulated  in  this  way  to 
maintain  a  constant  potential  for  variable  currents  in  the 
external  circuit;  but  a  simple  calculation  for  an  actual 
case  will  show  that  in  order  to  maintain  the  constant  po- 
tential for  a  small  current  in  the  external  circuit,  the  field 
magnet  coils  would  have  to  be  wound  with  comparative 


196  Principles  of  Dynamo- Electric  Machines. 

small  wire,  and  that,  therefore,  very  much  current  would 
have  to  be  shunted  through  the  adjustable  resistance  when 
the  current  in  the  external  circuit  is  a  maximum.  This 
would  necessitate  resistances  of  large  size,  and  would 
waste  considerable  energy  when  the  machine  is  doing 
its  greatest  work.  Such  a  system  should,  therefore, 
if  used  at  all,  be  limited  to  cases  in  which  the  range 
of  adjustment  is  quite  small,  as  for  instance  to  ma- 
chines which  are  used  with  a  nearly  constant  load  of  lamps. 
The  advantage  of  such  machines  over  a  shunt  machine  is 
evidently  the  saving  in  the  cost  of  the  wire  and  winding  of 
the  magnet  coils.  For  a  comparatively  high  resistance 
armature  there  will  be  less  regulation  required  for  constant 
potential;  this  method  is,  therefore,  applicable  best  for 
Email,  cheap  machines  for  nearly  constant  loads. 

All  the  methods  of  regulation  described  may  be  made 
automatic  by  the  addition  of  a  proper  mechanism  which 
automatically  alters  the  resistances,  position  of  brushes, 
etc.  Such  regulators  consist  essentially  of  two  parts,  first, 
the  detector,  the  object  of  which  is  to  detect  changes  in 
the  potential  or  current  which  is  to  be  maintained  constant, 
and  second,  the  mechanism  which  is  actuated  by  this  de- 
tector and  which  effects  the  necessary  changes  in  the 
resistances,  sections  of  coils,  brushes,  or  speed.  Numerous 
more  or  less  effective  automatic  regulators  of  this  kind 
have  been  devised,  some  of  which  have  proved  to  be  very 
satisfactory.  As  they  have  been  described  in  detail  in 
text  books  and  periodicals,  it  is  not  necessary  to  repeat 
the  description  here. 


CHAPTER    X. 

Examining   Machines. 

AFTER  a  dynamo  is  completely  finished  it  is  well  to  sub- 
ject it  to  a  thorough  examination,  not  only  for  the  purpose 
of  being  assured  that  it  will  run  properly,  but  also  to  find 
out  whether  it  is  properly  proportioned  in  all  its  parts,  or 
whether  it  could  have  been  improved  in  some  of  its  pro- 
portions, and  if  so,  what  proportions  are  faulty,  and  to 
what  extent  they  could  have  been  improved.  It  will  like- 
wise show  whether  the  machine  is  run  under  the  most  ad- 
vantageous conditions,  and  if  not,  what  changes  would 
enable  it  to  be  run  so.  The  data  which  may  be  obtained 
from  such  examinations  when  properly  made,  will  often  be 
found  to  be  of  great  assistance  in  designing  other  machines 
of  different  sizes,  proportions  and  styles. 

As  these  tests  may  sometimes  show  faulty  construction 
or  proportioning,  it  is  well  to  make  some  of  them  while 
the  machine  is  wound  with  temporary  field  magnet  coils, 
that  is,  before  it  is  finally  wound  with  its  proper  coils. 
This  applies  more  particularly  to  the  saturation  and  the 
heating  tests,  as  also  to  the  test  for  the  effect  of  the  counter 
magnetization  of  the  armature.  Some  of  these  tests  will, 
of  course,  be  different  according  as  the  machine  is  separ- 
ately excited  with  temporary  coils  and  an  exciter,  or 
whether  it  is  self-exciting,  as  a  shunt,  series  or  compound 
machine. 

Among  the  most  important  of  these  tests  is  the  deter- 
mination of  what  are  called  the  "  characteristics  "  of  the 
machine.  These  are  the  results  of  a  succession  of  tests 
plotted  in  the  form  of  a  curve,  which  then  shows  graphi- 
cally some  of  the  more  important  features  of  the  machine. 
Such  curves  will  show  at  a  glance  how  one  quantity,  say 

(197) 


198 


Principles  of  Dynamo-Electric  Machines. 


the  electromotive  force  or  current,  will  change,  when 
changes  are  made  in  another  quantity,  say  the  external  re- 
sistance, speed,  or  magnetization.  For  instance,  let  a 
series  wound  machine  be  run  at  a  constant  speed,  and  let 
it  discharge  through  a  succession  of  different  resistances 
(which  need  not  be  measured)  varying  from  a  very  large 
one  to  as  small  a  one  as  the  machine  will  stand  without 
injury.  Measure  both  the  difference  of  potential  at  the 
terminals,  and  the  current,  for  each  resistance.  On  a  piece 
of  cross-section  paper  lay  off  these  different  currents  to 
any  convenient  scale,  along  the  horizontal  line  o  x,  figure 
37,  from  o  to  the  right;  similarly  lay  off  the  differences 


2000; 
< 


1000- 


5  10  15  20         Y 

Fig.37. 

SERIES  MACHINE. 

of  potential  from  o  to  Y  on  the  same  or  on  any  other  con- 
venient scale.  The  intersections  of  the  vertical  and  hori- 
zontal lines  drawn  through  each  two  corresponding  values 
of  the  current  and  difference  of  potential  respectively,  will 
give  a  series  of  points,  the  curved  line  o  a  b  c  through 
which  will  be  the  characteristic  for  the  current  and  potential 
of  that  machine.  This  curve  shows  at  a  glance  that  when 
the  currents  are  small  the  potential  increases  rapidly  with 
an  increase  of  current  ;  and  as  the  first  part  of  the  curve 
is  a  straight  line  it  shows  that  the  potential  increases  in 


Examining  Machines.  199 

the  same  proportion  as  the  current.  When  the  current 
has  been  increased  to  a  certain  value,  in  this  case  ten 
amperes,  the  potential  no  longer  increases  so  rapidly,  and 
at  fifteen  amperes  it  has  reached  a  maximum  limit ;  for 
still  greater  currents  the  potential  falls  again.  From  this 
characteristic  the  following  deductions  can  be  made. 
Barring  other  considerations,  this  machine  should  not  be 
run  normally  for  currents  or  potential  less  than  those  of 
the  point  a,  for  if  it  is,  the  curve  shows  that  it  is  not  run- 
ning to  the  best  advantage,  or  in  other  words,  the  machine 
is  larger  than  it  need  be.  It  should  not  be  run  much  be- 
yond the  point  5,  as  that  \*  the  maximum  point  for  the 
potential,  and  it  is  probable  that  the  machine  heats  greatly 
beyond  this  point,  and  therefore  runs  at  a  disadvantage 
and  with  poor  efficiency.  Furthermore,  for  values  of  the 
currents  between  a  and  c  the  machine  will  give  a  nearly 
constant  potential  for  different  loads,  the  variations  in  the 
potential  being  less  the  nearer  this  part  a  c  of  the  curve 
approaches  a  horizontal,  straight  line.  The  curvature  at 
a  is  due  to  the  iron  having  become  saturated,  and  shows 
that  a  greater  current  in  the  series  coils  than  that  for  a 
has  little  effect  in  increasing  the  magnetism,  arid  therefore, 
the  potential;  series  wound  machines  for  arc  lights  in 
series  may  therefore  be  run  above  the  point  a,  because 
changes  in  the  current  due  to  poor  regulation  of  the  lamps, 
will  then  have  less  effect  in  causing  the  potential  to  change, 
in  other  words,  the  magnets  are  less  sensitive  to  changes 
of  current  beyond  the  point  a,  and,  therefore,  the  current 
will  be  more  nearly  constant. 

In  the  same  way  the  characteristic  may  be  drawn  be- 
tween the  known  external  resistance  and  the  potential, 
showing  how  the  latter  will  vary  for  different  external 
resistances.  Similarly,  that  for  the  resistance  and  current 
will  show  how  the  latter  will  vary  with  changes  in  the 
former,  and  between  what  values  of  the  resistance  the 
current  will  be  nearly  constant.  By  running  the  machine 
at. different  speeds  and  discharging  through  the  same  fixed 


200 


Principles  of  Dynamo-Electric  Machines. 


resistance,  the  characteristic  for  the  speed  and  current,  or 
speed  and  potential  may  be  obtained.  Or  if  the  resistance 
is  also  varied  for  each  speed  a  succession  of  curves,  like 
that  in  figure  87  will  be  obtained,  showing  the  effect  of 
changes  of  speed.  But  in  general,  characteristics  involv- 
ing changes  of  the  speed,  though  interesting  and  instructive, 
are  of  less  practical  value,  as  it  is  assumed  at  the  outset 
that  the  speed  is  as  great  as  mechanical  considerations  will 
permit,  as  pointed  out  in  another  chapter.  The  speed 
characteristics  become  important  when  it  is  desired  to  shift 


m 


60  100  150        200 

Fig. 33. 

SHUNT  MACHINE. 

the   curved   part  a  b  of   the   characteristic   nearer  to  or 
further  from  the  point  o. 

A  characteristic  curve  can  be  obtained  for  any  two 
varying  quantities  which  depend  on  each  other,  and  for 
only  two,  as  for  instance,  the  current  and  potential ;  all 
other  quantities  (as  for  instance,  the  speed)  should  remain 
the  same,  except,  of  course,  such  quantities  as  the  external 
resistance  in  this  particular  case,  which  must  be  changed 
to  produce  a  change  of  current  and  potential.  If  it  is  de- 
sired to  vary  a  third  quantity,  such  as  speed  in  this  case, 
separate  curves  must  be  drawn  for  successive  values  of 
the  speed. 


Examining  Machines.  201 

Similar  characteristics  cannot  be  compared  with  one 
another,  unless  reduced  to  the  same  scales,  for  it  is  evi- 
dent that  two  characteristics  drawn  to  different  scales, 
may  be  identical  in  appearance  and  size,  but  still  repre- 
sent totally  different  results. 

The  general  characteristic  for  current  and  potential  of  a 
shunt  machine,  for  different  resistances,  is  shown  in  figure 
38.  Starting  first  with  open  circuit,  that  is  with  an 
infinitely  large  resistance,  and,  therefore,  no  current,  the 
machine  gives  its  maximum  potential  o  m  /  with  a  grad- 
ually decreasing  resistance  the  current  increases  and  the 
potential  falls  slightly  ;  it  will  continue  to  fall  propor- 
tionately to  the  current  as  the  resistant  decreases,  as  far  as 
the  line  m  a  is  a,  straight  line.  At  a  the  potential  falls 
rapidly,  and  at  b  the  maximum  current  is  reached ;  by 
decreasing  the  resistance  still  more,  the  current  and  po- 
tential both  fall  until  the  resistance  is  zero,  that  is,  until 
the  machine  is  short  circuited,  when  both  are  zero.  This 
shows  that  there  is  a  maximum  current  o  e,  which  can 
be  obtained  from  a  shunt  machine,  and  that  the  maximum 
potential  o  m,  is  when  the  machine  is  run  on  open  circuit. 
It,  furthermore,  shows  that  if  the  armature  can  stand  the 
maximum  current  o  e,  then  no  adjustment  of  the  external 
resistance  will  injure  the  machine.  This  is  not  the  case 
with  a  series  machine  which  would  soon  be  destroyed 
when  run  on  too  small  a  resistance,  as  seen  from  its  char- 
acteristic. A  shunt  machine  is  never  run  in  practice  be- 
tween the  points  b  and  o,  because,  as  could  readily  be 
proved,  the  same  output  could  be  obtained  from  the  same 
armature  and  frame  with  a  much  less  expensive  winding 
of  the  field  magnets;  it  would  therefore  be  very  uneconom- 
ical, as  far  as  first  cost  is  concerned,  to  run  a  shunt 
machine  at  the  part  b  o  of  the  curve.  The  best  machines 
are  seldom  run  near  the  point  b,  being  usually  limited  to 
a  short  part  of  the  line  m  a.  It  would  not  be  safe,  there- 
fore, to  diminish  the  external  resistance  too  much  for  fear 


202  Principles  of  Dynamo-Electric  Machines. 

the   maximum   current  o  e  might  be  too  great   for  the 
armature. 

Jt  will  be  noticed  that  the  part  m  a  is  nearly  straight, 
and  that,  therefore,  the  potential  falls  regularly  but  slowly 
even  for  wide  ranges  of  the  current,  and  that  if  this  line 
could  be  made  to  be  horizontal  the  potential  would  be  con- 
stant for  the  corresponding  values  of  the  current  from  o  to 
of.  The  inclination  of  this  line  is  due  to  the  fact  that 
some  of  the  electromotive  force  is  absorbed  in  the  arma- 
ture itself  to  overcome  its  internal  resistance,  and  that  this 
amount  increases  with  the  current.  If,  therefore,  the 
armature  resistance  be  practically  zero,  there  would  be  no 
fall  of  potential  in  the  armature,  and,  therefore,  this  line 
m  a  would  be  horizontal,  in  other  words,  the  shunt  ma- 
chine would  have  a  constant  potential  for  greatly  varying 
currents.  This,  however,  would  necessitate  having  either 
a  very  large  armature  or  a  very  intense  field,  both  of  which 
add  to  the  cost  of  a  machine.  Instead  of  avoiding  this 
fall  of  potential,  it  may  be  neutralized  or  balanced  by  a 
simple  combination  of  the  two  characteristics,  figures  38 
and  37,  that  is  by  a  combined  shunt  and  series  winding. 
In  figure  38  the  potential  at  first  falls  in  a  regular  propor- 
tion, while  in  figure  37,  it  rises  in  a  regular  proportion, 
therefore,  by  a  proper  combination  of  the  two,  their  total 
action  may  be  made  to  keep  the  potential  constant.  To 
do  this  the  part  o  a  of  the  series  curve  should  make  only 
a  small  angle  with  the  horizontal,  as  o  a,  figure  39,  which  t 
is  accomplished  by  using  only  a  few  series  windings.  This 
angle  should  be  such  that  the  increase  of  potential  ^  a, 
figure  39,  due  to  the  series  coils  for  any  current,  of,  should 
be  equal  to  the  fall  of  potential,  g  a',  due  to  the  shunt  coils, 
for  which  m  a'  is  the  curve.  The  resulting  action  of  the 
two  will,  therefore,  be  a  constant  potential  for  all  currents 
from  zero  to  a  certain  limit,  of.  The  characteristic  for 
this  compound  machine  will  therefore  be  m  g.  Plow  to 
proportion  these  two  sets  of  coils  to  obtain  this  result  has 
already  been  explained  in  chapter  viii.  It  will  be  noticed 


Examining  Machines. 


203 


that  the  shunt  characteristic  in  figure  39  curves  down- 
wards for  currents  greater  than  o/j  and,  that  therefore, 
that  for  the  compound  machine  m  g,  will  also  curve  down- 
ward beyond  g.  This  shows  that  a  compound  machine 
should  be  used  only  in  such  parts  of  the  two  characteristics, 
o  a  and  m  a',  as  are  nearly  straight  lines,  for  up  to  this 
limit  only,  can  it  be  made  to  have  a  constant  potential  for 
different  currents.  It  can  readily  be  shown  that  the  curv- 
ing of  the  line  m  g  at  g,  is  due  chiefly  to  the  magnetic 
parts  of  a  machine  being  nearly  saturated  ;  a  compound 


771 


Fig. 39. 

COMPOUND  MACHINE. 

machine    should  therefore  be  run  below  the  saturation 
point,  as  explained  in  chapter  viii. 

This  leads  to  another  and  very  important  test  wjiich 
machines  should  be  subjected  to,  namely,  the  test  for 
saturation.  This  test  is  best  made  with  temporary  coils  on 
the  magnets,  excited  by  a  separate  exciter  the  current 
from  which  can  be  varied  between  wide  limits.  It  can 
also  be  made  with  the  finished  coils  of  a  machine  if  the 
exciter  has  the  proper  potential  for  these  coils.  It  can 
also  be  made  with  shunt  or  series  machines  by  letting 
them  excite  themselves,  but  this  is  generally  less  satis- 


204 


Principles  of  Dynamo-Electric  Machines. 


factory,  at  least  when  it  is  desired  to  test  the  magnetic 
qualities  independently,  especially  for  series  machines,  as 
it  is  preferable  and  in  some  cases  essential  to  have  no 
appreciable  current  flowing  in  the  armature. 

To  conduct  this  test,  run  the  machine  on  open  circuit  at 
a  constant  speed  and  excite  the  magnets  with  currents 
varying  successively  from  a  very  small  current  to  as  great 
a  one  as  the  magnet  coils  will  stand.  Set  the  brushes  to 
the  position  of  greatest  potential  and  measure  in  each  case 


EXCITING   CURRENT 


Fig.40. 

SATURATION  CURVE. 

the  potential  at  the  brushes  and  the  exciting  current.  This 
potential  will  then  be  a  correct  measure  of  the  useful 
magnetism  of  the  field  for  each  case,  while  the  exciting 
current  will  represent  the  cost  of  generating  this  magnet- 
ism. Lay  off  these  values  to  any  convenient  scale  and  find 
the  characteristic  curve  as  shown  in  figure  40.  It  will 
generally  be  found  that  this  curve  resembles  somewhat 
that  of  a  series  machine,  figure  37,  being  at  first  nearly  a 
straight  line,  showing  that  the  magnetism  increases  rapidly 
with  the  exciting  current  ;  it  then  curves,  showing  that  it 
no  longer  increases  rapidly,  or  in  other  words,  that  the  iron 
has  become  saturated.  If  this  curving  of  the  line  is  short 
and  decided  as  in  o  ay  it  shows  that  the  iron  parts,  including 


Examining  Machines.  205 

cores,  armature  and  yoke  pieces,  are  all  saturated  about 
the  same  time,  and  are  therefore  properly  proportioned  as 
far  as  saturation  is  concerned.  If,  on  the  other  hand,  the 
curve  is  gradual  with  little  or  no  straight  part,  as  o  b,  it 
shows  that  some  of  the  iron  parts  are  saturated  before  the 
others.  This,  in  some  cases,  as  for  compound  machine,  is 
objectionable,  and  should  be  remedied  by  increasing  the 
iron  parts  of  smallest  cross-section,  except  when  the  ma- 
chine is  to  be  magnetized  only  as  far  as  the  curve  is  tolera- 
bly straight.  Whenever  the  smallest  cross-section  of  the 
iron  parts  is  in  the  yoke  piece,  it  should,  of  course,  be 
remedied  for  any  machine,  but  wherever,  as  is  usually  the 
case,  it  is  in  the  cross-section  of  the  magnet  cores,  or  in 
Gramme  ring  machines,  in  the  armature  core,  it  is  a  ques- 
tion easily  determined  by  trial  calculations,  whether  the 
gain  in  the  economy  of  magnetism  obtained  by  increasing 
this  cross-section,  is  justified  by  the  additional  expense  of 
the  increased  amount  of  wire,  the  greater  size,  weight  and 
resistance  of  the  armature,  etc.,  which  this  necessitates. 

The  end  of  the  tolerably  straight  portion  of  this  char- 
acteristic shows  the  limit  at  which  magnetism  is  produced 
economically.  Whether  the  machine  (shunt  or  series,  but 
not  compound)  should  be  run  at  a  higher  degree  of  mag- 
netization can  readily  be  determined  by  trial  calculations ; 
it  evidently  depends  on  whether  the  gain  in  the  output  of 
the  machine  is  justified  by  the  poorer  magnetic  economy 
(i.  6.,  lower  efficiency  of  the  machine)  and  the  other 
objectionable  effects  of  over-saturation. 

Regarding  the  details  of  this  saturation  test,  it  is  evi- 
dent that  a  reliable  voltmeter  with  a  large  range  is  required. 
It  is  not  necessary  to  know  the  actual  values  of  thereadings 
in  volts,  but  it  is  necessary  to  know  the  relative  values,  that 
is,  the  voltmeter  should  either  be  one  whose  deflections 
are  proportional  to  the  potential,  or  else  one  in  which  the 
relative  values  of  the  deflections  are  known.  If  the  volt- 
meter has  only  a  small  range  of  readings  the  machine  may 
be  run  at  different  speeds,  the  speed  being  made  lower  as 


206  Principles  of  Dynamo  Electric  Machines. 

the  exciting  current  increases.  To  correct  the  readings 
for  this  change  of  speed,  it  is  sufficiently  accurate  to 
assume  that  the  voltage  is  proportional  to  the  speed;  this 
enables  all  the  readings  to  be  reduced  to  one  speed,  and 
applies  therefor  as  well  to  the  first  method  if  the  speed 
during  that  test  has  varied  slightly.  Instead  of  changing 
the  speed  to  enable  a  voltmeter  of  small  range  to  be  used, 
successive  resistances  equal  to  that  of  the  voltmeter  may 
be  used  in  series  with  it,  thus  reducing  the  readings  to  one 
half,  one-third,  etc. ;  but  this  is  not  to  be  recommended,  as 
these  resistances  should  not  only  be  equal  to  that  of  the 
voltmeter,  but  also  have  the  same  self-induction,  and  un- 
less they  are  carefully  made  it  is  difficult  to  make  both  the 
resistance  and  self-induction  the  same.  If  the  voltmeter 
has  no  appreciable  self-induction  this  objection  does  not 
apply.  In  the  absence  of  a  suitable  voltmeter  any  indi- 
cator of  small  currents,  as  for  instance  a  test  galvanometer, 
may  be  used  together  with  a  suitable  adjustable  resistance, 
such  for  instance  as  the  plug  resistances  box  accompanying 
a  Wheatstone  bridge.  Connect  the  resistances  in  series 
with  the  test  galvanometer  and  adjust  them  so  that  any 
suitable  deflection  is  obtained  for  the  first  reading.  For 
any  other  potential  adjust  the  resistances  until  the  deflec- 
tion is  reduced  to  the  same  reading,  then,  knowing  the  re- 
sistance of  the  galvanometer,  the  potentials  will  be  pro- 
portional to  the  total  resistances,  including  that  of  the 
galvanometer  and  that  in  the  resistance  box. 

If  this  saturation  test  is  made  with  a  shunt  machine, 
that  is,  self -exciting,  the  speed  should  be  somewhat  greater 
than  the  normal  in  order  to  get  the  characteristic  curve 
through  the  saturation  point.  For  the  same  reason  if  the 
machine  to  be  tested  is  series  wound  it  should  be  tested 
for  a  greater  current  than  the  normal,  but  its  speed  may 
be  much  less,  the  external  resistance  being  made  smaller. 
It  is  preferable  in  this  case  to  keep  the  armature  current 
constant,  and  therefore  vary  the  exciting  current  by  vari- 
ably shunting  the  current  around  the  magnets. 


Examining  Machines.  207 

In  this  saturation  test  the  exciting  currents  multiplied 
by  the  number  of  windings  in  the  exciting  coils  gives  the 
ampere  turns.  These  might  have  been  laid  off  along  the 
horizontal  line  in  figure  40  in  place  of  the  exciting  current, 
but  as  the  number  of  windings  is  constant  it  would  be 
equivalent  merely  to  changing  the  scale  of  the  diagram, 
but  would  not  alter  the  relative  proportions  between  dif- 
ferent parts  of  the  curve.  If  the  coils  used  in  this  test  are 
different  from  those  to  be  used  finally,  it  is  necessary,  of 
course,  to  reduce  the  exciting  current  to  ampere  turns,  in 
order  to  eliminate  the  two  unlike  windings. 

The  numbers  obtained  from  this  saturation  test  will 
often  be  found  to  be  of  great  assistance  in  designing  other 
machines,  or  in  correcting  the  proportions  of  a  faultily 
constructed  one.  In  chapter  vii  some  of  the  applications 
of  this  data  has  been  described,  as  for  instance  in  correct- 
ing the  proportions  of  an  over-saturated  frame.  As  de- 
scribed there,  the  exciting  power  of  the  coils  (that  is,  the 
intensity  of  the  field  in  the  coil  if  there  was  no  iron  core) 
may  be  readily  calculated  approximately  from  the  ampere 
turns  and  the  cross-section  of  the  core  space.  The  con- 
stant thus  obtained  can  be  used  as  a  guide  in  calculating 
the  size  of  the  cores  or  the  ampere  turns  for  other  magnets 
in  which  the  exciting  power  is  to  be  of  the  same  intensity. 
By  calculating  the  total  number  of  lines  of  force  generated 
by  the  coils  themselves  (without  iron  cores)  as  described 
in  chapter  vii,  and  comparing  this  with  the  total  number 
of  useful  lines  of  force  in  the  armature  field,  found  by  calcu- 
lation from  a  test  (see  chapter  v  and  appendix  i),  constants 
may  be  obtained  between  the  actual  number  of  lines  of  force 
in  the  field,  and  the  ampere-feet  (that  is,  the  number  and 
length  of  the  ampere  turns)  which  generate  the  same.  This 
constant  serves  as  a  valuable  guide  for  determining  the  coil 
space,  ampere-feet  and  ampere  turns  in  designing 
other  machines.  There  are,  of  course,  other  factors  which 
enter  into  such  calculations  for  new  machines,  for  instance 
the  quality  of  the  iron,  the  magnetic  resistance  due  to  dif- 


308 


Principles  of  Dynamo-Electric  Machines. 


ferent  lengths  of  cores,  yoke  pieces,  etc.,  or  to  different  air 
spaces  between  the  field  and  the  armature  core;  these  will 
modify  the  calculations  somewhat,  and  would  complicate 
them  very  greatly  if  introduced,  but  as  they  are  of  second- 
ary importance  they  may  be  neglected  in  approximate  cal- 
culations, unless  they  are  too  widely  different  in  the  two 
machines.  As  in  almost  all  other  calculations  which  en- 
gineers have  to  make  in  designing  any  structure,  a  suitably 
chosen  "factor  of  safety"  will  cover  all  such  inaccuracies. 
A  careful  study  and  analysis  of  the  characteristic  curves 
of  a  machine  may  often  be  the  means  of  finding  out  faults 


10 


0  f 

Fig.4'1. 

SEPERATELY  EXCITED  MACHINE. 


in  the  proportioning  of  the  parts,  or  of  indicating  the  most 
favorable  conditions  of  running,  and  it  is  therefore  to  be 
recommended.  Such  curves  are  to  the  electrical  engineer 
as  the  curves  of  a  steam  indicator  cards  are  to  the  mechan- 
ical engineer,  and  there  is  no  doubt  that  the  latter  have 
been  the  means  of  making  a  much  more  careful  study  of 
the  steam  engine. 

The  characteristics  fo*  the  current  and  potential  of  the 
principal  forms  of  machines,  namely  series,  shunt  and 
compound,  have  been  shown  in  figures  37,  38  and  39,  that 
for  the  less  frequently  used  form,  the  separately  excited 


Examining  Machines.  209 

machine,  is  shown  in  figure  41.  The  exciting  current  being 
constant  the  magnetism  of  the  field  magnets  will  be  con- 
stant; the  potential  will  therefore  be  a  maximum  on  open 
circuit,  that  is  for  the  point  m,  when  there  is  no  current. 
As  the  current  from  the  machine  increases  (for  decreasing 
external  resistances)  the  potential  will  fall  slightly,  which 
is  evidently  due  to  the  internal  resistance  of  the  armature 
which  consumes  an  amount  of  potential  proportional  to  the 
current.  The  first  portion,  m  a,  of  this  curve  is  therefore 
straight  and  slightly  inclined  to  the  horizontal.  When  the 
armature  current  becomes  very  great,  and  especially  when 
there  are  many  armature  windings,  the  counter  magnetism 
of  the  armature  will  become  great  as  compared  to  the 
magnetism  of  the  field;  this  will,  by  diminishing  the  useful 
magnetism,  cause  the  potential  to  fall  more  rapidly,  and 
the  curve  will  then  no  longer  approximate  to  a  straight 
]ine,  as  shown  beyond  the  point  a.  As  this  counter  mag- 
netism generally  necessitates  shifting  the  brushes,  well 
built  machines  are  usually  run  with  currents  less  than  that 
for  the  point  a  where  this  rapid  fall  of  potential  begins. 
The  fall  of  potential  in  the  armature  due  to  its  internal  re- 
sistance, as  for  instance  a  g,  divided  by  the  total  electro- 
motive sorce  ^/"and  multiplied  by  100  will  evidently  give 
the  percentage  of  energy  lost  in  the  armature.  The  con- 
tinuation of  curve  m  a,  if  it  were  possible  to  determine  it, 
would  ultimately  meet  the  line  of,  at  a  point  which  repre- 
sents the  greatest  current  which  the  machine  gives  on 
being  short  circuited,  that  is,  when  the  potential  at  the 
poles  is  zero  and  the  total  electromotive  force  of  the  arma- 
ture is  absorbed  in  overcoming  the  internal  resistance. 
This  portion  of  the  curve,  although  of  interest  theoreti- 
cally, is  of  no  use  in  practice. 

Besides  determining  and  examining  the  characteristics 
and  the  saturation  curve  of  a  machine,  tests  may  also  be 
made  to  determine  other  qualities,  such  as  the  counter 
magnetization  of  the  armature,  the  shifting  of  the  brushes, 
the  exploration  of  the  field,  the  magnetic  leakage,  the  re- 


210  Principles  of  Dynamo- Electric  Machines. 

sistance  of  the  armature,  heating  of  armature  and  field, 
etc.,  some  of  the  results  of  which  tests  may  be  of  use  also 
for  determining  co-efficients  which  may  serve  as  guides  in 
designing  other  machines. 

The  effect  of  the  counter  magnetization  of  the  armature 
may  be  ascertained  by  exciting  the  field  with  a  constant 
current  from  some  outside  source  and  finding,  with  the 
aid  of  a  voltmeter  attached  to  the  brushes,  the  position 
which  the  brushes  must  have  to  give  the  greatest  poten- 
tial first  on  open  circuit  and  then  for  the  greatest  arma- 
ture current;  the  amount  of  the  change  of  the  position  of 
the  brushes  indicates  the  effect  of  the  counter  magnetiza- 
tion of  the  armature.  • 

The  effect  of  shifting  the  brushes  may  be  determined  by 
separately  exciting  the  magnets  and  moving  the  brushes 
to  successive  positions  to  the  right  or  left  of  the  neutral  line, 
measuring  the  potential  in  each  case.  This  may  be 
done  for  open  circuit,  for  various  current  strengths,  or  vari- 
ous degrees  of  saturation ;  the  same  test  may  also  be  made 
with  self-exciting  machines. 

To  explore  the  field  surrounding  the  armature,  the  fol- 
lowing simple  method  may  be  used.  Wind  a  single  coil 
of  one  or  more  turns  of  fine  wire  around  one  part  of  the 
armature,  over  or  next  to  one  of  the  regular  coils;  connect 
one  end  of  this  coil  to  the  shaft  and  the  other  to  a  little  in- 
sulated metallic  block  fastened  on  to  a  wooden  ring  secured 
around  the  commutator,  so  that  it  acts  as  a  single  commuta- 
tor bar  for  this  little  coil.  Connect  the  terminals  of  a  sensi- 
tive voltmeter  or  galvanometer  to  two  brushes,  and  apply 
these  to  the  two  ends  of  this  coil,  one  on  the  shaft  and  one 
on  the  wooden  ring  containing  the  metallic  block.  When 
the  machine  is  running  normally  this  little  coil  will  generate 
an  electromotive  force,  and  therefore  a  current,  which  is 
proportional  to  the  strength  of  that  portion  of  the  field 
through  which  it  is  passing  when  the  brush  is  in  contact 
writh  the  block.  The  reading  of  the  voltmeter  or  galvano- 
meter will  therefore  indicate  the  strength  of  the  field.  By 


Examining  Machines.  211 

moving  that  brush  which  is  resting  on  the  wooden  ring- 
around  to  successive  positions  on  the  circumference  of  the 
ring,  the  relative  strengths  of  different  parts  of  the  field  may 
be  determined,  showing  whether  it  is  unbalanced,  and  if 
so  where  and  how  much,  also  where  the  neutral  line  is, 
how  wide  the  neutral  space  is,  how  great  the  magnetic  lag 
is,  etc.  This  test  may  be  made  first,  when  there  is  no  cur- 
rent  in  the  armature,  and  second,  when  the  normal  current 
is  flowing,  and  it  will  then  also  show  the  effect  of  the 
counter  magnetization  of  the  armature.  The  results  may 
be  plotted  in  the  form  or  characteristic. 

The  magnetic  leakage  may  be  found  by  simply  explor- 
ing the  space  surrounding  the  machine  and  armature  by 
means  of  a  compass  needle,  the  direction  of  which  will  in- 
dicate the  direction  and  position  of  the  lost  lines  of  f  orce.: 
To  determine  this  leakage  quantitatively,  make  a  small, 
thin  loop  of  one  or  more  turns  of  fine  wire,  connect  its 
terminals  to  a  sensitive  galvanometer  having  a  moderately 
heavy  needle  so  as  to  act  as  a  ballistic  galvanometer. 
When  the  machine  is  excited  normally  from  an  external 
source,  the  armature  being  at  rest,  place  this  little  coil  or 
loop  to  its  full  length  in  the  space  between  the  armature 
and  the  pole-piece,  and  withdraw  it  very  rapidly;  it  will 
cut  the  lines  of  force  and  develop  an  electromotive  force 
which  will  be  proportional  to  the  current  or  deflection  of 
the  galvanometer.  By  similarly  moving  this  same  coil 
through  the  lines  of  force  of  the  leakage,  its  plane  being 
always  perpendicular  to  these  lines  of  force,  the  de- 
flections of  the  galvanometer  will  indicate  the  intensities 
of  the  leakage  at  different  places,  as  compared  to  the  in- 
tensity of  the  useful  field,  and  will  therefore  give  the  per- 
centage of  leakage.  If  the  actual  useful  number  of  lines 
of  force  per  square  inch  of  the  armature  field  has  been  cal- 
culated as  described  from  the  induction  in  the  armature, 
the  actual  number  of  lines  of  force  per  square  inch  of  the 
leakage  is  readily  determined. 
1.  See  Appendix  m. 


212  Principles  of  Dynamo-  Electric  Machines. 

The  armature  resistance  may  be  measured  by  any  con- 
venient method  of  measuring  small  resistances,  one  of  the 
best  of  which  is  to  measure  the  fall  of  potential  by  the 
potentiometer  method,1  when  a  known  current  is  sent 
through  the  armature,  or  to  compare  its  resistance  with  a 
known  resistance  in  series  with  it,  by  the  fall  of 
potential  in  each.2  When  a  reliable  volt  and  ampere 
meter  are  at  hand,  a  very  simple  way  of  measur- 
ing it  is  to  measure  the  potential  first  on  open  cir- 
cuit while  running,  and  separately  excited,  and  then 
on  closed  circuit  with  a  current  which  is  not  great 
enough  to  distort  the  field;  the  difference  of  the  two  po- 
tentials divided  by  the  current  will  then  be  the  resistance 
of  the  armature.  If  the  exact  length  and  diameter  of  the 
armature  wire  is  known  the  resistance  may  be  calculated, 
remembering  that  it  is  one-fourth  of  that  of  the  whole 
wire. 

To  find  the  heating  coefficients,  measure  the  temperature 
by  placing  the  bulb  of  a  thermometer  on  the  magnet  coils 
after  running  normally  for  at  least  three  or  four  hours,  and 
covering  the  bulb  with  a  little  cotton  ;  measure,  also,  the 
radiating  surface  of  the  coils  and  the  number  of  watts 
dissipated  in  them  ;  from  these  the  coefficient  may  then 
be  calculated  as  described  in  chapter  viii.  To  measure 
the  same  for  the  armature  it  will  be  sufficiently  accurate 
for  all  practical  purposes  to  hold  the  thermometer  in 
the  air  currents  from  the  armature,  between  the  pole- 
pieces,  or  else  to  place  it  on  the  armature  immediately  af- 
ter stopping,  and  covering  the  bulb  as  before  with  cotton. 

The  efficiency  of  a  machine  is  determined  from  the  horse- 
power applied  at  the  pulley,  and  the  electrical  power  de- 
veloped. There  are  several  ways  of  stating  this  efficiency. 
The  real  or  true  commercial  efficiency  is  the  useful  en- 
ergy delivered  in  the  external  circuit  divided  by  that  ap- 
plied to  the  pulley  ;  this  in  the  best  machines  varies  from 

1.  See  Flemings1  Short  Lectures  to  Electrical  Artisans,  page  129. 

2.  See  Flemings'  Short  Lectures  to  Electrical  Artisans,  page  143. 


Examining  Machines.  213 

80  per  cent,  to  91  or  92  per  cent.,  and  evidently  takes  into 
account  all  losses  in  the  machine.  The  other  efficiency, 
and  the  one  often  given  by  the  makers,  and  called  the 
total  efficiency  or  efficiency  of  conversion,  is  the  total  elec- 
trical energy  divided  by  that  applied  to  the  pulley,  the 
total  electrical  energy  being  that  in  the  external  circuit, 
that  lost  in  the  armature  and  that  lost  in  the  field.  It  is 
evidently  higher  than  the  other,  being  sometimes  as  higli 
as  97  per  cent.  The  "economic  coefficient"  is  the  first 
efficiency  divided  by  the  second.  In  measuring  the  effi- 
ciency as  well  as  in  all  cases  in  which  it  is  necessary  to 
measure  the  actual  potential  accurately,  care  should  be 
taken  in  selecting  the  voltmeter,  for  if  the  voltmeter  is  in 
the  form  of  a  coil  having  considerable  self-induction,  the 
readings  for  the  same  potential  will  be  quite  different  (as 
much  as  25  per  cent,  it  is  claimed)  according  to  whether 
the  machine  gives  a  steady  current  (as  a  unipolar  machine 
or  battery),  or  whether  its  current  pulsates  as  in  any  ma- 
chine with  a  commutator.  The  value  of  a  reading  there- 
fore depends  on  the  number  of  commutator  bars  and  the 
speed.  The  most  reliable  method  is  to  use  a  sensitive  re- 
flecting galvanometer  with  coils  of  few  windings,  calibrate 
it  with  a  standard  cell,  and  when  using  it  with  the  high 
potentials  of  a  dynamo  add  high  resistances  in  the  galva- 
nometer circuit  sufficient  to  get  proper  deflections.  The 
constants  of  the  instrument  in  volts  per  degree  of  deflec- 
tion, will  then  be  proportional  to  the  total  resistances  in 
the  galvanometer  circuit  in  the  two  cases. 


APPENDIX   I. 


Practical   Deductions  from  the  Franklin  Institute  Tests 
of  Dynamos. 

THE  tests  of  dynamo-electric  machines  made  in  1885 
under  the  auspices  of  the  Franklin  Institute,  are  un- 
doubtedly the  most  reliable  and  complete  of  all  the  impar- 
tial tests  which  have  ever  been  made  and  published,  and 
they  therefore  afford  the  practical  electrical  engineer  an 
excellent  opportunity  to  deduce  proportions,  dimensions 
and  constants,  to  assist  him  in  designing  dynamos,  espec- 
ially as  the  machines  which  were  tested  are  among  the 
best  that  are  made,  and  represent  the  results  of  tedious 
and  expensive  experimenting  on  the  part  of  the  makers, 
while,  at  the  same  time,  they  embody  the  improvements 
suggested  by  long  and  continued  use  of  the  machines  in 
practice. 

Dynamos  have  frequently  been  built  by  "  guessing  "  at 
the  proportions,  constructing  them,  and  then  trying  them 
to  "see  what  they  will  give."  If  they  then  turn  out  (by 
chance)  to  give  the  electromotive  force  and  current  de- 
sired, the  designer  generally  gets  the  credit  for  having 
made  very  correct  "calculations;"  while  if  they  give,  for 
some  unknown  reason,  quite  different  results,  the  manu- 
facturer has  to  be  consoled  with  the  statement  that  "  it  is 
not  possible  to  calculate  the  parts  of  a  dynamo." 

If  a  machine  is  at  hand  which  can  be  thoroughly  tested 
and  measured  in  all  its  parts,  it  is  not  difficult  for  a  tech- 
nical engineer,  who  is  well  informed  concerning  the  prin- 
ciples and  practice  of  dynamo  building,  to  calculate  the 
parts  of  another  machine  of  the  same  type  which  will  give 
a  certain  desired  electromotive  force  and  current  slightly 
different  from  that  of  the  first  machine.  But  when  the 
designer  has  no  access  to  such  a  model  machine,  or  if  the 

(215) 


216  Practical  Deductions  from  the 

current  and  potential  desired  differ  greatly  from  those  of 
the  model,  it  is  difficult,  if  not  quite  impossible,  to  calcu- 
late with  any  degree  of  certainty  the  parts  of  a  dynamo 
from  the  principles  and  laws  of  induction  and  resistance, 
without  some  practical  constants  and  proportions,  which 
can  be  obtained  only  from  existing  machines. 

As  an  aid  and  guide  in  such  calculations,  a  set  of  prac- 
tical constants  and  proportions  have  been  calculated  by  the 
writer  from  the  valuable  tests  of  the  Franklin  Institute. 
These  constants  will  not  only  materially  diminish  the 
amount  of  "  guessing  "  in  designing  machines,  but  it  is  be- 
lieved they  are  sufficiently  complete  to  enable  a  technical 
engineer  to  calculate  all  the  electrical  proportions  of  a  well- 
designed  cylindrical  armature  which  is  to  give  a  certain 
required  electromotive  force  and  current.  It  is  to  be  re- 
gretted that  the  data  given  in  the  Franklin  Institute  Re- 
port are  not  sufficient  to  enable  a  similar  complete  set  of 
constants  to  be  deduced  for  the  field.  A  few  of  these  may, 
however,  be  calculated,  and  they  will  materially  aid  in  de- 
termining certain  parts  of  the  field.  The  calculations  of 
armatures  being  based  on  induction  and  conductivity,  the 
constants  from  one  armature  may  be  used  in  calculating 
others;  but  in  the  field  this  is  not  the  case,  as  the  relation 
between  the  exciting  current  and  the  magnetism  produced 
depends  very  largely  on  the  size,  shape  and  proportions,  of 
the  coils  and  the  iron  parts,  including  the  cores,  the  so- 
called  neutral  parts,  pole  pieces,  etc.,  as  well  as  on  the  qual- 
ity of  the  iron.  As  these  are  so  very  different  in  different 
machines,  the  constants  obtained  from  one  would  be  of 
little  use  in  determining  the  size  of  other  fields,  except, 
perhaps,  the  constants  given  below  for  the  intensity  and 
amount  of  the  magnetism  of  the  field.  A  careful  and  sys- 
tematic builder  of  dynamos  will  in  most  cases  run  the  arma- 
ture, when  completed,  with  its  own  field  magnet  frame 
with  temporary,  removable  coils  of  known  number  of 
turns,  on  the  magnets,  in  order  to  find,  by  regulating  the  cur- 
rent in  these  temporary  field  coils,  how  many  ampere-turns 


Franklin,  Institute  Tests  of  Dynamos.  217 

are  required  in  the  field  to  induce  the  desired  electro- 
motive force  and  current  in  the  armature.  From  this  num- 
ber of  ampere-turns  any  competent  dynamo  builder  can 
readily  calculate  the  proper  windings  of  the  magnets  with 
all  due  accuracy. 

Numerous  theories  of  dynamo-electric  machines  have 
been  advanced,  but  most  of  them  are  of  greater  interest  to 
the  mathematician  and  the  physicist  than  to  the  practical 
dynamo  builder.  More  attention  seems  to  have  been  given 
to  integrals,  complicated  abstruse  fractions  and  formulae 
for  determining  the  presumably  correct  values  for  extreme 
impossible  cases,  than  to  simple,  practical  formulae,  by 
means  of  which  tangible  results  may  be  obtained  for  the 
ordinary  forms  of  machines.  In  some  cases,  formulae  based 
on  physical  theories  have  been  suggested,  which  might 
have  been  of  practical  use  if  accompanied  by  the  absolute 
and  reliable  values  of  certain  constants  usually  repre- 
sented by  Greek  letters,  which  enter  as  direct  factors 
in  the  formulae;  but  the  values  of  these  constants  have 
either  been  omitted,  or  else  have  been  given  within 
such  wide  ranges  that  they  cannot  be  used  to  any  advan- 
tage in  practice.  They  are  sometimes  of  such  a  nature 
that  they  differ  for  each  machine,  thus  necessitating  the 
construction  and  testing  of  the  machine  before  the  values 
of  the  constants  for  calculating  this  particular  machine 
may  be  determined ;  which  is,  to  say  the  least,  a  very  awk- 
ward way  of  applying  a  formula. 

The  writer  has,  therefore,  set  aside  all  abstruse  theories 
of  the  dynamo,  which  have  yet  to  stand  the  severe  test  of 
varied  practical  application,  and  has  endeavored  to  deduce 
from  the  results  given  in  the  Franklin  Institute  Report  some 
constants,  or  values,  which  are  in  such  a  form  that  they  may 
be  directly  applied  to  the  calculations  of  the  armature 
and  some  parts  of  the  field.  The  deductions  of  these  con- 
stants are  based  only  on  the  well-known  laws  of  induction, 
and  of  mechanics,  and  as  they  are  calculated  from  actual 
cases,  they  show  what  is  done  in  practice,  as  distinguished 


218  Practical  Deductions  from  the 

from  what  might  be  done  according  to  some  theory,  pro- 
vided the  theory  is  correct. 

The  results  contained  in  the  accompanying  table  not  only 
give  the  values  which  can  be  used  in  designing  machines,  but 
an  attempt  has  also  been  made  to  determine  the  efficiency  of 
separate  parts;  thus  indicating  under  what  circumstances 
the  most  advantageous  proportions  may  be  arrived  at.  Some 
proportions  used  in  calculating  the  numbers  given  in  the 
table  were  not  contained  in  the  report,  and,  therefore,  had  to 
be  estimated,  except  where  they  were  furnished  to  the  writer 
by  the  kindness  of  the  manufacturers.  But  the  errors  which 
may  have  been  introduced  by  a  slightly  inaccurate  esti- 
mate, are  so  small  that  they  do  not  materially  affect  the 
results.  Certain  slight  errors,  or  modifications,  in  some  of 
the  figures,  or  proportions,  were  not  taken  into  account,  as 
they  cannot  be  determined;  but  they  are  of  such  a  nature 
as  to  affect  equally,'  or  approximately  so,  all  machines  not 
differing  too  widely  from  the  style  of  those  tested.  Among 
these  is  the  self-induction  of  the  armature;  in  well-built 
machines,  like  those  tested  in  which  the  field  is  intense,  the 
speed  not  too  high,  the  number  of  commutator  bars  or  coils 
large,  and  the  number  of  windings  per  coil  very  small,  the 
self-induction  will  be  very  small,  and,  therefore,  the  dif- 
ference between  the  self-induction  in  these  different  arma- 
tures may  be  neglected. 

In  the  table  given  below,  the  deduced  constants  have 
been  accompanied  by  many  of  the  proportions  from  which 
they  were  derived,  as  copied  from  the  report,  in  order  to 
show  some  of  the  principal  proportions  of  the  machines, 
and  to  give  the  conditions  under  which  the  constants  have 
the  values  given.  Numerous  other  values  in  the  report 
might  have  been  repeated  here,  but  as  it  is  presumed  that 
any  one  can  obtain  the  original  report,  they  are  omitted. 
The  values  chosen  for  the  deductions  were  taken  from  that 
one  of  the  full  load  tests  in  which  the  current  and  poten- 
tial were  nearest  to  the  values  given  by  the  makers  as  the 
best  working  load. 


Franklin  Institute  Tests  of  Dynamos.  219 

The  following  assumptions  were  made,  as  the  accurate 
data  for  deducing  the  same  were  wanting.  The  speed  in 
the  Edison  No.  4  machine  was  not  given  in  the  report;  that 
given  here  is  the  speed  at  which  the  makers  say  the  ma- 
chine is  to  be  run,  and  it  is  assumed  that  the  speed  must 
have  been  very  nearly  this  in  the  test.  The  percentage  of 
the  whole  circumference  of  the  armature  which  is  embraced 
by  the  pole  pieces  was,  in  the  Weston  machines,  assumed 
to  be  about  80$,  as  this  was  the  proportion  in  some  arc 
light  machines  of  the  same  makers,  which  we  understand 
have  the  same  type  of  frame.  The  distance  between  the 
pole-piece  projections  was  determined  from  this  in  the 
Weston  machines.  In  the  Edison  machines  the  length  of 
the  pole  pieces  and  the  armature  core,  were  deduced  from 
the  statement  in  the  report  giving  the  length  of  useful  wire 
in  a  coil;  it  was  assumed  that  by  the  term,  "useful  wire," 
was  meant  that  which  lies  directly  between  the  pole  pieces 
and  "the  armature  core.  In  the  Edison  machines  it  was 
assumed  that  the  length  of  the  armature  core  was  the  same 
as  that  of  the  pole  pieces;  in  the  Weston  machines,  this 
was  the  case. 

As  the  electromotive  force  induced  in  an  armature  is  de- 
pendent upon  the  amount  of  magnetism  passed  through 
per  second,  the  first  questions  in  designing  armatures  are: 
How  great  may  the  velocity  of  the  moving  wire  be  ?  What 
must  its  length  be  ?  What  must  the  intensity  of  the  field 
be?  etc. 

The  first  of  these,  the  velocity  of  the  moving  wire — com- 
monly called  the  "  conductor  or  inductor  velocity  " — de- 
pends on  the  distance  of  the  active  wire  from  the  centre  of 
the  shaft,  and  on  the  number  of  revolutions  of  the  arma- 
ture. It  may  be  calculated  from  the  speed  and  the  mean 
of  the  external  diameter  and  the  diameter  of  the  core.  In 
the  accompanying  table,  the  horizontal  column  a  contains 
the  external  diameters,  b  the  internal,  and  c  the  mean. 
From  this  mean  value  the  circumference  in  feet  was  calcu- 
lated, which,  when  multiplied  by  the  speeds  J,  and  reduced 


220  Practical  Deductions  from  the 

to  seconds,  gives  the  inductor  velocities  in  feet  per  second, 
in  column  e.  From  these  values,  it  is  seen  that  the  Edison 
armatures  have  a  higher  velocity  of  the  moving  wire;  also, 
that  it  is  preferable  to  obtain  the  high  inductor  velocity 
by  making  the  diameter  as  great  as  practical,  rather  than 
to  increase  the  number  of  revolutions,  as  will  be  seen  by 
comparing  the  small,  high-speed  armature  of  the  Edison 
No.  4  with  the  large  one  of  the  Nos.  10  and  20. 

To  obtain  a  constant  by  means  of  which  the  length  of 
wire  may  be  calculated,  the  total  electromotive  force  in 
volts  which  was  generated,  has  been  divided  by  the  length 
of  that  part  of  the  wire  in  which  it  is  generated.  This 
wire  will  be  termed  the  "active  wire"  and,  in  these  de- 
ductions, has  been  limited  to  that  portion  of  the  armature 
wire  which  lies  directly  between  the  pole  pieces  and  the 
armature  core.  Strictly  speaking  this  is  not  quite  correct, 
as  some  induction  does  undoubtedly  take  place  in  some 
parts  of  the  wire  lying  at  the  ends  of  the  armature,  and 
also  in  some  of  the  longitudinal  wires  which  are  not 
directly  between  the  armature  core  and  the  pole  pieces;  but 
the  induction  in  both  of  these  parts  is  presumably  so  small 
as  compared  to  that  in  other  parts,  that  it  may  be  neg- 
lected, especially  as  the  constants  are  to  be  applied  to  sim- 
ilar armatures,  thus  eliminating  this  error.  The  length 
of  the  active  wires  were  calculated  from  the  following 
proportions.  Column /'gives  the  number  of  coils  or  com- 
mutator bars  ;  g  the  number  of  turns  per  coil,  and  h  the 
resulting  total  number  of  turns  on  the  armature.  The  pro- 
portion of  these  coils  which  are  active,  is  calculated  from 
the  distance  between  the  pole  piece  projections,  column  i, 
and  the  circumference  determined  from  the  diameters  in 
column  a  •  the  ratio  of  this  is  given  in  column^*,  in  percent- 
age of  the  circumference  of  the  armature  which  is  active  ; 
this  will  also  represent  the  percentage  of  the  number  of 
windings  which  are  active.  Column  k  gives  the  length  of 
a  pole  piece  in  inches,  which  is  the  same  as  the  length  of 
the  armature  core.  From  these  the  total  length  of  the 


Franklin  Institute  Tests  of  Dynamos.  221 

active  wire  in  feet  can  be  calculated,  remembering  that 
every  turn  of  wire  on  the  armature  represents  twice  the 
length  of  the  pole  piece  ;  but  as  the  two  halves  of  the  arm- 
ature wires  are  in  multiple  arc  only  half  of  this  induces 
the  whole  electromotive  force.  The  total  electromotive 
force  in  volts,  given  in  column  I,  is  then  divided  by  this 
half  length  of  active  wire  in  feet,  giving  the  induction  in 
volts  per  foot  in  column  m.  As  this  induction  is  slightly 
different  in  different  positions  of  the  wire  with  reference 
to  the  pole  pieces,  these  results  give  the  mean  value. 
They  show  that  the  induction  is  considerably  higher  in 
the  Edison  than  in  the  Weston  machines  ;  also,  that  it  is 
very  nearly  the  same  in  all  the  Edison,  and  nearly  the  same 
in  the  three  Weston  machines. 

These  constants  are  dependent  on  the  velocity  of  the 
inductor,  for  it  is  evident  that  if  the  latter  were  higher 
the  induction  would  be  increased.  In  order,  therefore,  to 
properly  compare  these  constants,  it  is  necessary  to  elim- 
inate the  velocity  by  dividing  these  figures  by  the  veloci- 
ties in  column  ey  thus  giving  the  volts  per  foot  which 
would  be  induced  (in  the  respective  fields)  at  a  uniform 
velocity  of  one  foot  per  second.  This  is  given  in  column 
n,  and  shows  the  number  of  volts  induced  per  foot  for  an 
inductor  velocity  of  one  foot  per  second.  These  may  be 
compared  with  each  other  as  the  velocities  are  the  same. 
They  show  that  at  the  same  velocity  the  induction  is  better 
in  the  Edison  than  in  the  Weston  machines  ;  also,  that  the 
values  agree  very  closely  for  all  the  Edison,  except  for  the 
No.  4  machine,  which  may  possibly  be  attributed  to  the 
number  assumed  for  the  speed,  which  was  not  taken  in  the 
test.  For  the  Weston  6  M  it  is  lowest,  which  is  no  doubt 
due  to  a  less  intense  field. 

The  next  question  which  naturally  arises  is,  what  was  the 
intensity  of  the  field  in  these  machines.  This  may  be  determ- 
ined as  follows  :  We  know  that  a  volt  is  100,000,000. 
times  the  unit  of  electromotive  force  in  the  absolute  sys- 
tem, and  also  that  one  absolute  unit  of  electromotive  force 


222  Practical  Deductions  from  the 

is  generated  when  a  wire  cuts  lines  of  force  at  the  rate  of 
one  per  second  ;  therefore,  one  volt  is  induced  if  a  wire 
cuts  100,000,000.  lines  of  force  per  second.  As  we  know 
the  number  of  volts  in  one  foot  (column  m)  in  these  ma- 
chines, and  also  the  number  of  feet  moved  through  in 
one  second  (column  e),  we  can  readily  calculate  the 
intensity  which  the  field  must  have  had  to  induce  that 
number  of  volts  in  one  foot.  To  illustrate  this,  sup- 
pose the  velocity  was  one  foot  per  second,  and  that 
the  induction  was  one  volt  per  foot  at  this  velocity, 
then  it  is  evident  that  the  surface  moved  over  by  one  foot 
of  wire  in  one  second,  that  is,  one  square  foot,  must  con- 
tain 100,000,000.  lines  of  force  ;  or,  if  the  induction  was 
two  volts  per  foot,  there  must  be  200,000,000.  lines  of  force 
in  this  space,  as  this  number  must  have  been  cut  per  sec- 
ond to  generate  two  volts.  Dividing  this  number  by  144, 
will  give  the  mean  intensity  of  the  field  in  number  or  lines 
of  force  per  square  inch.  These  figures  are  given  in  col- 
umn o.  They  show  that  with  one  exception,  all  the  Edison 
machines  have  very  nearly  the  same  intensity  of  field  ; 
also,  that  it  is  somewhat  lower  in  the  Weston,  especially  in 
the  6  M  machine  ;  but,  as  will  be  seen,  it  is  more  economi- 
cally obtained  in  this  one,  which  indicates  that  it  was 
probably  not  over  saturated. 

These  values  might  also  have  been  obtained  by  multi- 
plying those  in  column  n  by  100,000,000.  and  dividing  by 
144,  which  is  equivalent  to  multiplying  them  by  694,444. 
This  will  be  seen  by  considering  the  principles  of  the 
deductions. 

The  total  useful  amount  of  magnetism  in  the  whole  field 
is  evidently  the  intensity  per  square  inch  multiplied  by 
the  size  of  the  field  in  square  inches.  As  the  intensity 
has  been  calculated  from  the  amount  of  induction,  the 
amount  of  magnetism  thus  obtained  does  not  include  the 
leakage  of  the  magnetism,  that  is,  those  lines  of  force 
which  are  not  cut  by  the  armature  wire,  it  therefore  rep- 
resents the  useful  magnetism  only.  The  same  lines  of 


Franklin  Institute  Tests  of  Dynamos.  223 

force  which  enter  the  armature  at  one  pole  piece  pass 
through  it  to  the  other  pole  piece,  and  therefore  the  total 
number  of  lines  of  force  is  the  intensity  multiplied  by  the 
curved  area  of  one  pole  piece.  These  figures  are  given  in 
column  p,  and  are  deduced  from  columns  o,  a,  j  and  k. 
The  magnetism  increases  with  the  number  of  volt-amperes 
which  the  machine  generates. 

This  amount  of  magnetism  is  generated  at  the  expense 
of  a  certain  quantity  of  electrical  energy  in  the  field  mag- 
nets. In  order  to  get  some  approximate  values  for  calcu- 
lating roughly  how  much  electrical  energy  will  be  required 
for  generating,  in  practice,  a  certain  amount  of  magnetism 
in  similar  fields,  the  figures  in  column  p,  may  be  divided 
by  the  amount  of  energy  in  volt-amperes,  which  was  re- 
quired in  these  cases  to  generate  the  respective  fields.  In 
the  report  the  figures  in  column  <?,  are  given,  which  are 
the  energy  in  horse-power  consumed  in  the  fields.  Unfor- 
tunately the  resistance  of  the  field  without  the  regulator  box 
was  measured  only  in  one  machine,  so  that  the  energy  given 
here  represents  more  than  that  used  in  the  field  magnets 
themselves,  thus  introducing  an  error  in  all  the  deductions 
made  from  these  values.  It  is  presumed,  however,  that  at 
full  load  the  resistance  in  the  box  was  not  large,  so  that 
the  error  will  probably  be  small.  Reducing  this  energy  in 
the  field  to  volt-amperes  and  dividing  it  into  the  amount 
of  magnetism,  gives  the  number  of  useful  lines  of  force 
generated  per  volt-ampere  in  the  field.  These  are  given 
in  column  r.  As.  they  depend  on  so  many  different  pro- 
portions of  the  parts  of  the  field  and  magnet  coils,  and  also 
in  a  measure  on  the  armature,  they  might  vary  considera- 
bly for  different  types  of  field  magnets,  and  can,  therefore, 
be  used  only  in  making  rough  preliminary  calculations. 
They  agree  tolerably  well  for  these  fields,  except  for 
the  Weston  6  M,  which  is  evidently  better  proportioned 
than  the  others.  As  these  figures  are  the  ratio  of  that 
which  is  produced  to  that  which  is  required  to  produce  it, 
thev  may  be  said  to  represent  the  relative  efficiencies  of 


224  Practical  Deductions  from  the 

the  different  fields,  in  which  sense  they  represent  no  abso- 
lute efficiencies,  but  serve  simply  to  compare  the  efficiencies 
with  one  another.  In  the  Edison  machines  we  believe 
wrought  iron  alone  is  used,  while  in  the  Weston  both 
wrought  and  cast  iron  were  formerly  used  together,  and 
we  presume  were  used  in  these  machines  also.  Possibly 
the  iron  in  the  field  of  the  Weston  6  M  machine  is  not 
over  saturated,  which  may  be  the  reason  of  its  high  effi- 
ciency of  field. 

The  wires  of  the  armature  being  wound  repeatedly 
around  it,  pass  through  the  same  field  a  number  of  times, 
thus  utilizing  the  same  field  in  one  revolution  for  succes- 
sive inductions  in  the  same  wire.  This  number  of  turns,  as 
fiven  in  column  A,  multiplied  by  column  J,  may,  therefore, 
e  said  to  represent  the  economic  use  of  the  field  ;  it  does 
not  follow,  however,  that  the  best  armature  is  the  one  hav- 
ing the  greatest  number  of  turns  on  it,  as  it  is  quite  the 
reverse,  other  more  important  considerations,  such  as 
self-induction,  sparking,  etc.,  require  that  the  number  of 
turns  of  wire  be  as  small  as  practicable. 

In  deducing  the  number  of  volts  per  foot,  only  the  active 
wire  was  taken  into  consideration.  In  designing  arma- 
tures it  is  therefore  necessary  to  know  what  the  proportion 
is  between  the  active  and  the  total  length  of  the  wire,  for 
if  the  active  length  is  determined  first  from  the  number  of 
volts  to  be  generated  and  the  induction  per  foot,  we  must 
find  what  the  total  length  is  in  order  to  determine  the  re- 
sistance or  cross-section  of  the  wire.  This  proportion  of 
active  to  total  length  will  evidently  depend  on  several  di- 
mensions of  an  armature,  and  will,  therefore,  vary  some- 
what in  different  machines.  For  these  machines  it  was 
determined  as  follows  :  column  s  contains  the  mean  length 
of  wire  in  one  turn  (where  several  smaller  wires  are  in 
parallel  they  were  considered  as  one)  ;  this  multiplied  by 
the  number  of  coils  in  column/,  gives  the  total  lengths  in 
column  t  ;  these  divided  into  the  active  length  in  column  ?«, 
give  the  percentages  in  column  v.  From  the  latter  it  will 


Franklin  Institute  Tests  of  Dynamos.  225 

be  seen  that  the  economy  of  the  wire  is  considerably  better 
in  the  Edison  than  in  the  Weston> an^ tnat  ^  ™  ^>est  i11  tne 
Edison  No.  20,  the  proportion  between  the  length  and  di- 
ameter of  the  armature  being  greatest  in  this  one.  Most 
of  the  proportions  from  which  these  percentages  have  been 
deduced  were  given  by  the  makers. 

The  density  of  the  current  in  the  armature  wire  can  be 
determined  from  the  diameters  of  the  wire  in  column  w,  the 
number  of  wires  in  parallel  in  a  coil,  in  column  35,  and  the 
total  current  in  the  armature  in  column  y.  Dividing  the  lat- 
ter into  the  double  area  of  cross-section  of  the  wire  in  square 
mils,  or  the  double  sum  of  the  cross-sections  of  the  parallel 
wires,  gives  the  number  of  square  mils  per  ampere  in  col- 
umn z.  These  numbers  vary  somewhat  with  the  current 
and  with  several  other  proportions  ;  they  should,  therefore, 
be  used  only  as  a  general  guide,  or  for  making  preliminary 
calculations.  The  best  method  for  determining  the  cross- 
section,  is  to  find  the  resistance  from  the  amount  of  energy 
which  is  allowed  to  be  lost  in  the  armature,  which  together 
with  the  total  length  of  the  wire  as  determined  from  the 
induction  per  foot,  etc.,  gives  the  required  cross-section. 

The  percentage  of  energy  in  the  armature  as  taken  from 
the  report,  is  given  in  column  A.  Those  in  which  the  loss 
is  least,  Weston  6WI  and  Edison  No.  10,  have  the  greatest 
area  of  cross-section  of  wire  per  ampere  (column  2),  while  the 
one  in  which  the  loss  is  greatest,  Weston  7  M,  has  the  least 
cross-section  per  ampere.  This  relation  does  not  necessa- 
rily exist  between  them  all,  as  the  percentage  of  loss  de- 
pends also  on  the  induction  per  foot  of  wire,  and  therefore 
on  the  length  of  the  wire. 

The  relative  efficiencies  of  these  armatures,  as  inductors, 
may  be  seen  from  the  figures  in  column  G.  The  efficiency 
as  an  inductor,  apart  from  that  as  a  converter  of  energy, 
is  greater  the  larger  the  total  amount  of  electrical  energy 
induced  (column  JB),  and  the  less  the  amount  of  wire 
necessary  to  effect  this  induction  ;  it  may,  therefore,  be  ex- 
pressed by  the  quotient  of  the  two.  The  figures  in  column 


226  Practical  Deductions  from  the 

C  have,  therefore,  been  calculated  by  dividing  those 
in  column  B  by  the  lengths  in  column  t  and  by  the  double 
area  of  cross-section  determined  from  columns  w  and  x,  the 
decimal  point  being  changed  to  reduce  them  to  a  conven- 
ient form.  By  themselves  these  figures  represent  nothing, 
but  by  comparing  them  with  one  another  they  show  which 
of  the  armatures  are  the  best  proportioned  as  inductors. 
They  show  that  the  Edison  are  considerably  better  propor- 
tioned than  the  Weston.  The  most  efficient  armature  is, 
strange  to  say,  the  smallest  one,  Edison  No.  4,  which  is  no 
doubt  due  to  its  having  the  most  intense  field  (column  o), 
the  highest  speed  (column  d),  and  almost  the  smallest 
cross-section  of  wire  per  ampere  (column  z) ;  this  higher  effi- 
ciency is,  however,  obtained  at  the  expense  of  the  efficiency 
of  the  field,  as  will  be  seen  from  column  r  ;  the  useful  com- 
mercial efficiency  of  the  whole  machine  is,  therefore,  below 
the  average,  as  seen  from  column  D.  Next  to  this  arma- 
ture, in  point  of  efficiency  as  an  inductor,  is  the  largest 
one,  Edison  No.  20  ;  its  high  efficiency  is  no  doubt  due  to 
the  great  intensity  of  field  (column  o),  the  very  small 
amount  of  wire  on  the  armature  (column  £),  the  relatively 
high  inductor  velocity  (column  0),  the  large  proportion  of 
active  wire  (column  v\  and  the  comparatively  small  cross- 
section  of  the  wire,  per  ampere  (column  z\  The  field  of 
this  machine  has  next  to  the  highest  efficiency  as  seen  in 
column  r,  and  therefore,  as  might  be  inferred,  the  machine 
has  the  best  useful  commercial  efficiency  of  all  of  those 
tested,  as  seen  in  column  D.  It  may  be  interesting  to  men- 
tion here  that  in  this  machine  which  has  the  best  efficiency, 
the  armature  wires  cut  the  field  less  frequently  than  in  any 
other,  as  seen  from  the  number  of  turns  in  column  A. 

Another  proportion  which  may  serve  as  a  guide  in  de- 
signing armatures,  is  the  relation  between  the  outside  di- 
ameter of  the  armature  and  the  diameter  of  the  core,  or 
what  amounts  to  the  same  thing,  the  percentage  of  the  ex- 
ternal diameter  which  is  taken  up  by  the  wire  on  both 
sides.  These  "figures  are  given  in  column  F>  and  are 


Franklin  Institute  Tests  of  Dynamos.  227 

obtained  by  dividing  twice  the  depth  of  the  windings  given 
in  column  JEloy  the  external  diameter  in  column  a.  This 
is  greatest  in  the  smallest  armature,  Edison  No.  4,  showing 
another  disadvantage  of  small  armatures  ;  it  is  least  and 
therefore  best,  in  the  Weston  6  M,  which  may  partially 
account  for  the  economic  field  shown  in  column  r,  as  the 
latter  depends  on  this  non-magnetic  space. 

A  few  other  proportions  may  be  deduced  from  the  data 
given.  One  of  these  is  the  proportion  of  the  distance 
between  the  pole-piece  projections  and  the  distance, 
between  the  pole  pieces  and  the  armature  core.  This  pro- 
portion should  evidently  be  as  great  as  practicable.  The 
figures  are  given  in  column  G-  and  are  obtained  from  col- 
umns i  and  E,  allowing  about  yV  of  an  inch  for  clearance  on 
each  side.  It  is  greatest  for  the  largest  machine,  Edison 
No.  20,  which  may  partially  account  for  its  economic 
field  in  column  r.  The  reciprocals  of  one-half  of  the  num- 
bers in  column  G-  may  be  said  to  represent  approximately 
the  intensity  of  the  leakage  of  magnetism,  as  compared  to 
the  intensity  of  the  useful  field  at  these  places,  as  the  lines 
of  force  have  the  choice  of  these  two  paths.  But  this 
will  be  only  a  rough  approximation  as  a  field  is  always 
more  intense  at  points  or  sharp  edges. 

Another  useful  proportion  is  the  relation  between  the 
length  and  diameter  of  the  armature  core,  as  deduced  from 
columns  k  and  b.  It  is  given  in  column  Hy  which  shows 
by  comparison  with  column  v  the  advantage  of  a  long  arma- 
ture. The  Weston  machines  appear  to  be  quite  uniform 
in  this  respect,  the  length  being  almost  twice  the  diameter. 

Column  I  gives  the  relation  between  the  length  and 
diameter  of  the  bearings. 

Column  J  gives  the  electromotive  force  between  two 
neighboring  commutator  bars.  If  this  is  great  enough  to 
maintain  an  arc  across  the  insulation  of  the  commutator 
bars  (about  20  volts)  there  is  danger  of  starting  the  well- 
known  flash,  encircling  the  whole  commutator,  if  the 
brushes  should  be  misplaced  sufficiently  far  to  start  the 


228  Practical  Deductions  from,  the 

arc.  It  will  be  seen  to  be  far  within  the  limit  of  20  volts 
in  all  of  these  machines. 

The  Edison  Nos.  10  and  20  machines,  afford  a  good  op- 
portunity to  compare  two  armatures  of  different  lengths, 
but  having  all  the  other  sizes  and  proportions  alike,  in- 
cluding the  number  of  windings,  size  of  wire,  depth  of 
winding,  etc.,  only  that  in  the  one  three  wires  are  con- 
nected in  parallel,  and  in  the  other,  two  ;  and  that  in  the 
No.  20  the  distance  between  the  pole-piece  projections  is 
greater,  making  a  difference  of  about  10  per  cent,  in  the 
number  of  active  wires  in  favor  of  the  smaller  one.  The 
chief  gain  of  the  long  armature  is  seen  in  column  v,  in 
which  it  has  the  highest  percentage,  which  partially  ac- 
counts for  the  efficiency  in  column  C,  and  the  consequent 
commercial  efficiency  in  column  D. 

The  subject  of  self-induction  of  the  armature  was  pur- 
posely omitted  here,  partly  because  insufficient  data  are 
given  in  the  report  to  make  any  practical  deductions 
regarding  it,  but  principally  because  it  is  presumed  that  the 
self-induction  in  such  armatures  as  these,  jvith  very  few 
turns,  is  so  small  that  it  may  be  neglected  in  such  rough 
values  as  have  been  deduced  here.  It  is  to  be  regretted 
that  the  resistance  of  the  field  coils  without  the  regulator 
box  was  not  measured  in  each  case,  in  order  that  the  effect 
of  the  self-induction  of  the  field  coils  in  increasing  the  ap- 
parent resistance  could  be  calculated,  and  its  relation  to 
the  number  of  pulsations  or  commutator  segments  determ- 
ined. In  only  one  case  was  the  field  resistance  measured 
alone,  and  from  this  a  calculation  shows  an  increase  of  re- 
sistance of  less  than  one-tenth  of  one  per  cent.,  and,  there- 
fore, probably  less  than  the  allowable  error  in  measurement. 

The  fact  that  the  Edison  Nos.  5  and  20  machines  gave 
way  in  the  armature  insulation,  will  not  lessen  the  value'  of 
these  deductions  to  the  designer  of  dynamos,  as  this  was 
presumably  a  defect  in  the  details  of  construction  and  not 
in  the  proportion  of  parts. 

The  test  of  the  20-light  machines  were  marked  unofficial 


Franklin  institute  Tests  of  Dynamos.  22U 

in  the  report  because  the  preliminary  run  of  ten  hours  was 
not  on  full  load  ;  but  they  are  no  doubt  sufficiently  accur- 
ate for  the  deductions  of  the  approximate  values  in  the 
table. 

The  deductions  made  here  are  not  for  the  purpose  of 
comparing  the  commercial  value  of  these  machines,  as  it 
has  been  shown  in  the  tests  that  they  are  practically  equal 
electrically.  The  most  important  consideration  in  this  re- 
spect would  be  the  cost  of  the  machines  and  the  cost  of 
maintenance  and  attendance,  which  cannot  be  considered 
here. 

For  the  benefit  of  any  one  not  familiar  with  these  ma- 
chines, it  may  be  added  that  they  are  all  simple  shunt  ma- 
chines, with  cylinder  armatures. 

The  full  report  of  these  tests  will  be  found  in  the  sup- 
plement to  the  Journal  of  the  Franklin  Institute,  Novem- 
ber, 1885. 

In  conclusion,  it  may  be  said  that  the  values  deduced  in 
the  table  may  not  be  free  from  small  errors,  as  great  ac- 
curacy was  not  possible  in  all  cases,  owing  to  the  want  of 
some  few  detail  dimensions  ;  moreover,  great  accuracy  has 
no  particular  value  in  such  general  deductions  as  these. 


230 


Deductions  from  Franklin  Institute  Tests. 


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APPENDIX   II. 


The  So-called  "Dead  Wire"  on   Gramme  Armatures. 

THE  question  whether  the  wire  on  the  inside  of  the 
Gramme  ring  armature  and  on  the  ends  of  a  cylinder 
armature,  is  active  or  inactive,  has  been  discussed  at  great 
length  in  the  last  few  years,  but  apparently  without  lead- 
ing to  any  definite  and  indisputable  conclusions.  Recent 
publications  show  that  some  authorities  still  appear  to  be- 
lieve that  for  some  theoretical  reasons  the  wire  on  the 
inside  of  a  Gramme  ring  is  as  active  as  that  on  the  outside. 
The  writer  has  therefore  made  a  simple  and  conclusive  ex- 
periment which  appears  to  prove  beyond  question, 
whether  this  wire  is  active  or  inactive. 

The  question  is  not  one  which  is  of  interest  merely  from 
a  theoretical  point  of  view,  but  it  bears  directly  on  im- 
portant points  in  the  construction  of  machines,  such  for 
instance  as  placing  a  magnet  in  the  inside  of  a  Gramme 
ring,  or  making  a  cylinder  armature  of  square  section 
instead  of  oblong. 

The  two  theories  which  explain  the  generation  of  elec- 
tromotive force  by  moving  a  wire  near  a  magnet,  may  be 
briefly  summarized  as  follows  :  If  a  loop  of  wire,  abed 
in  figure  1,  be  revolved  about  its  axis  in  a  magnetic  field 
between  two  opposite  poles,  an  electromotive  force  will  be 
generated  in  the  direction  as  shown.  The  first  and  older 
theory,  sometimes  termed  the  theory  of  threading  lines  of 
force  through  a  loop,  explains  the  induction  of  an  electromo- 
tive force,  by  saying  that  the  number  of  lines  of  force  which 
pass  through  this  revolving  loop  is  always  either  increas- 
ing or  decreasing,  and  that  this  increase  or  decrease  in  the 
number  of  lines  of  force  threaded  through  the  loop  gen- 
erates electromotive  force.  According  to  the  second  and 

(231) 


232 


The  So-called  "Dead  Wire' 


cutting 


newer  theory,  generally  termed  the  theory  of 
lines  of  force,  an  electromotive  force  is  generated  when  a 
wire  cuts  lines  of  force  ;  the  induction  therefore  takes 
place  in  the  two  parts  e  f  and  g  h  of  the  loop  a  b  c  <7,  and 
only  in  those  parts,  as  the  remainder  of  the  loop  does  not 
cut  lines  of  force. 

It  will  be  seen  that  these  theories,  as  stated,  do  not  con- 
flict, both  may  be,  and  undoubtedly  are,  correct  as  far  as 


they  go.  The  second  theory  is,  however,  by  far  the  more 
satisfactory  as  it  tells  us  precisely  where  the  induction 
takes  place,  and  where  we  must  endeavor  to  place  the 
wire  of  a  loop  or  coil  in  order  to  render  it  active.  The 
first  theory,  referring  only  to  the  loop  in  general,  leaves  us 
to  believe  that  the  loop  must  be  considered  as  a  whole,  and 
that,  therefore,  all  parts  of  the  wire  forming  the  loop  are  of 
equal  importance  in  the  generation  of  electromotive  force, 
that  is,  they  are  all  equally  active.  The  theory  does  not 
state,  directly,  that  all  parts  of  the  loop  are  equally  active. 


On  Gramme  Armatures.  233 

but  it  leaves  one  to  infer  that  this  is  the  case  ;  it  is  this  in- 
terpretation of  the  theory  which,  it  is  claimed,  proves  that 
that  part  of  the  wire  which  is  on  the  inside  of  the  Gramme 
ring  and  on  the  ends  of  a  cylinder  armature  is  as  active  in 
the  generation  of  electromotive  force  as  the  other  parts 
of  it. 

This  interpretation  of  the  older  theory  necessarily  con- 
flicts with  the  theory  of  cutting  lines  of  force  ;  both  cannot 
be  correct.  We  believe  that  the  older  theory  is  generally 
interpreted  in  this  way  ;  if  not,  and  if  the  induction  is  not 
the  same  in  all  parts  of  the  loop,  then  the  theory  is  not 
sufficiently  complete  or  exact  to  enable  us  to  properly  de- 
sign dynamos,  as  it  does  not  tell  us  where  the  induction 
does  take  place.  According  to  the  older  theory  it  is  the 
loop  a  b  c  d  as  a  whole,  which  generates  the  electromotive 
force,  the  theory  does  not  tell  us  what  parts,  if  any,  of  the 
loops  are  superfluous.  On  the  other  hand,  the  second 
theory  states  that  the  induction  takes  place  in  the  parts 
e/and  g  A,  and  that  the  rest  of  the  loop  is  useless  as  an 
inductor,  it  serves  merely  as  a  conductor  or  collector  of 
the  currents,  and  should,  therefore,  be  made  as  short  as 
possible. 

In  order  to  prove  conclusively  which  of  these  two  con- 
flicting statements  is  wrong,  without  the  use  of  any  ab- 
struse theoretical  deductions,  the  writer  made  the  follow- 
ing simple  experiment  with  a  view,  more  particularly,  of 
finding  out  whether  the  wire  on  the  inside  of  a  Gramme 
ring  was  active  in  generating  electromotive  force,  or 
whether  it  was  mere  dead  resistance,  acting  only  as  a 
conductor. 

The  core  of  a  Gramme  ring  was  wound  with  a  number 
of  coils,  each  one  consisting  of  only  one  turn  of  a  wire,  as 
shown  in  figure  2,  in  which  E  R  is  the  core  of  the  ring, 
shown  in  vertical  cross-section  ;  N,  s,  are  the  two  poles  of 
the  field,  and  c,  c,  c,  are  three  of  the  coils.  Each  coil  con- 
sisting of  one  turn,  is  insulated  from  all  the  others 
and  has  its  two  ends,  I,  m,  bare  and  not  connected  with 


234 


The  So-called  "Dead  Wire" 


any  of  the  others  ;  each  coil  has  its  insulation  scraped  off 
at  the  bend  n.  Three  fixed  brushes,  I,  m,  w,  were  fastened 
to  the  frame  of  the  machine  so  as  to  touch  these  three 
parts  of  each  coil  as  it  passed  by  them.  The  field  was  ex- 
cited by  another  machine  and  the  armature  was  made  to 
revolve  rapidly.  In  applying  a  test  galvanometer  to  the 


brushes  I  and  m,  as  shown,  there  was  a  strong  current 
flowing  in  the  direction  indicated,  showing  the  action,  of 
one  coil  of  a  simple  Gramme  armature.  In  order  to  find 
out  whether  this  induction  took  place  equally  in  all  parts 
of  the  loop  or  coil  In  m,  according  to  the  older  theory,  or 
whether  it  took  place  only  in  that  part  I  n  which  cuts  lines 
of  force,  the  test  galvanometer  was  placed  first  between 


On  Gramme  Armatures.  £35 

the  brushes  I  and  n,  indicating  a  current,  as  shown,  in  the 
same  direction  as  before,  the  current  indicated  by  it  being 
in  this  case  only  that  which  was  generated  in  the  half  of 
the  loop  I  n,  the  other  brush  m  having  first  been  removed. 
The  brush  I  was  then  removed  and  the  galvanometer 
placed  between  the  brushes  m  and  n,  in  which  case  it 
indicated  no  current  whatsoever,  showing  conclusively  that 
there  was  no  electromotive  force  generated  by  that  portion 
(m  n)  of  the  loop  which  was  in  the  inside  of  the 
ring.  According  to  the  older  theory  it  ought  to  assist  in 
generating  the  electromotive  force  manifested  at  I  and  m, 
as  it  forms  part  of  the  loop  I  n  m  through  which  the  lines 
of  force  are  threaded  from  the  pole-piece  N  to  the  core  E. 
The  experiment,  however,  showed  conclusively  that  it  did 
not  generate  electromotive  force,  that  all  the  current  which 
flowed  from  I  to  m  through  the  galvanometer  was  genera- 
ted in  the  part  n  I,  the  other  part,  n  m,  acting-merely  as  a 
dead  resistance  to  conduct  the  current  from  the  end  n  of 
the  active  wire  to  the  brush  m.  According  to  the  newer 
theory  the  part  In  is  the  only  portion  of  the  loop  which 
cuts  lines  of  force,  and  should,  therefore,  be  the  only  one 
in  which  induction  takes  place, — which  the  experiment  has 
shown  to  be  the  case.  The  part  in  n  being  shielded  from 
the  magnetism  by  the  iron  R,  does  not  cut  lines  of  force 
and  is,  therefore,  dead  resistance.  This,  therefore,  applies 
equally  well  to  the  wire  at  the  ends  of  cylinder  armatures. 
This  shows  conclusively  the  correctness  of  the  theory  of 
cutting  lines  of  force  and  the  fallacy  of  the  older  theory  as 
interpreted  above.  But  aside  from  theories,  it  shows  that, 
as  there  is  no  induction  in  the  part  m  n,  it  is  preferable  to 
dispense  with  it  if  practicable,  for  instance,  by  bringing  it 
around  on  the  outside  of  the  ring  on  the  opposite  side, 
thus  making  a  cylinder  armature  of  it,  or  by  rendering  it 
active  by  creating  a  field  in  the  inside  of  the  ring  with  an 
additional  pair  of  pole  pieces  and  magnets.  The  experi- 
ment also  shows  that  if  it  were  not  for  other  reasons  the 
best  form  for  a  cylinder  armature,  as  far  as  relates  to  dead 


236  The  So-called  "Dead  Wire" 

wire,  would  be  to  have  it  very  long,  and  very  small  in 
diameter. 

A  second  experiment  was  then  made  to  still  further 
prove  the  correctness  of  the  theory  of  cutting  lines  of  force. 
The  field  was  made  very  intense  so  as  to  over-saturate  the 
core  R  R,  thus  causing  some  lines  of  force,  a  a,  to  leak  through 
the  air  space  in  the  inside  of  the  ring  ;  such  conditions 
no  doubt  often  exist  in  Gramme  armatures,  the  iron 
in  the  core  of  the  ring  being  frequently  much  too  small  in 
cross-section.  In  again  applying  the  test  galvanometer  at 
I  and  m,  or  at  I  and  n,  the  same  results  as  before  were  ob- 
tained, but  on  connecting  it  with  m  and  n,  it  indicated  a 
current  in  the  direction  shown  by  the  dotted  arrows, 
which  is  in  the  reverse  direction,  around  the  coil,  to  that 
induced  in  n  I,  and  therefore  tends  to  neutralize  it.  This 
is  readily  explained  by  the  newer  theory,  as  the  wire  m  n 
cuts  the  lines  of  force  a  a  and,  therefore,  must  generate  a 
current  in  the  direction  shown.  The  older  theory  seems 
to  fail  completely  to  account  for  this  current,  for,  accord- 
ing to  that  theory  it  ought  to  be  in  the  other  direction. 
The  only  way  to  account  for  the  current  by  this  theory, 
appears  to  be  to  consider  the  loop  as  being  formed  by  the 
wire  m  n  and  the  test  galvanometer  circuit.  But  this 
fails  to  explain  why  the  difference  of  potential  exists  at  m 
and  n  when  there  is  no  galvanometer  circuit,  a  fact  which 
can  readily  be  proved. 

As  the  wire  on  the  inside  is  thus  shown  to  be  dead,  it 
might  be  inferred  that  the  advantage  of  placing  a  second  ' 
pole-piece  and  magnet  in  the  inside  would  be  very  great, 
doubling  the  number  of  volts  generated,  as  it  doubles  the 
active  length  of  wire.  But  this  is  by  no  means  the  case, 
provided  the  machines  are  in  both  cases  well  proportioned. 
The  reason  is  as  follows.  For  the  same  armature,  the  num- 
ber of  volts  generated  depends  only  on  the  total  number  of 
lines  of  force  cut,  provided  the  speed  is  the  same.  If  the 
armature  core  is  saturated  by  the  ordinary  form  of  pole 
pieces,  it  is  not  possible  to  economically  increase  to  any 


On  Gramme  Armatures.  237 

great  extent  the  number  of  lines  of  force  which  enter  it  ; 
a  second  pole-piece  may  be  placed  in  the  interior  and  will 
distribute  the  field  over  a  larger  surface  of  the  armature, 
but  it  cannot  add  much  more  magnetism  because  the  core, 
being  already  saturated,  cannot  be  forced  to  take  many 
more  lines  of  force.  It  therefore  appears  that  any  such 
gain  due  to  interior  pole  pieces  would  be  limited  to  the 
cases  when  the  armature  core  is  not  saturated  by  the 
external  pole  pieces.  But  there  is  apparently  no  reason 
why  this  case  should  occur,  as  it  is  always  possible  to 
saturate  the  small  cross-section  of  the  ring  by  the  large 
pole  pieces  on  the  outside. 

We  are  informed  by  good  authority  that  the  theory  of 
cutting  lines  of  force  assumes  that  the  number  of  lines  of 
force  which  follow  the  ring  from  pole  to  pole  is  propor- 
tional to  the  area  exposed  to  the  pole  pieces.  We  are 
unable  however  to  find  anywhere,  such  a  statement  as  a 
part  of  this  theory;  on  the  contrary  it  can  readily  be  proved 
that  this  general  assumption  is  not  correct  in  many  cases. 
As  long  as  the  iron  of  the  ring  is  below  saturation  then  it 
is  true  that  the  induction  will  increase  about  in  the  same 
proportion  as  an  increase  in  the  size  of  the  pole-piece  area, 
provided  the  intensity  per  square  inch  remains  the  same, 
for  it  is  evident  that  in  this  case  the  total  number  of  lines 
of  force,  that  is,  the  capacity  of  the  magnets,  will  have  to 
be  increased  proportionally.  But  as  soon  as  the  ring  is 
saturated,  then  an  increase  in  the  size  of  the  pole  pieces 
(of  the  same  intensity  per  square  inch)  will  no  longer  in- 
crease the  number  of  lines  of  force  proportionally.  In 
this  case,  which  is  the  more  common,  the  above  assumption 
is,  therefore,  not  correct.  It  is  understood,  of  course,  that 
if  the  area  of  the  pole  pieces  of  the  same  intensity  be 
doubled  for  instance,  the  capacity  of  the  magnets  for  gen- 
erating the  magnetism  must  be  doubled,  as  there  are  then 
twice  as  many  lines  of  force. 

This  subject  of  interior  pole  pieces  was  referred  to  in 
•me  of  the  writer's  earlier  articles  in  which  it  was  shown 


some 


238  The  So-called  "Dead  Wire" 

that  the  electromotive  force  will  be  increased  proportion- 
ally with  an  increase  in  the  number  of  lines  of  force  pass- 
ing into  the  core,  and  that  these  lines  of  force  may  be  in- 
creased by  increasing  the  size  of  the  pole-piece  surface  of 
the  same  intensity,  provided  the  iron  in  the  magnetic  cir- 
cuit is  not  over-saturated,  in  other  words  if  the  cross-section 
of  the  iron  (including  the  core  of  the  armature)  be  increased 
in  proportion  to  the  number  of  lines  of  force.  For  instance, 
suppose  the  cross-section  of  the  core  of  the  armature  to  be 
doubled,  and  the  number  of  lines  of  force  entering  it  be 
doubled  either  by  doubling  the  area  of  pole  pieces  of  the 
same  intensity,  or  by  doubling  the  intensity  and  keeping 
the  area  the  same,  then  it  is  evident  that  if  the  speed  with 
which  the  wire  passes  through  the  field  remains  the  same, 
the  induction  in  volts  will  be  twice  as  great,  and  as  the 
wire  is  not  necessarily  doubled  in  length  the  induction  in 
volts  per  foot  will  be  greater  than  it  was  before. 

As  the  small  core  of  a  Gramme  ring  can  generally  be 
saturated  by  pole  pieces  on  the  outside,  the  advantage  of 
interior  pole  pieces  must  not  be  looked  for  in  this  direction. 
The  chief  gain  seems  to  be  in  increasing  the  area  of  the 
non-magnetic  space  between  the  pole  pieces  and  the  arma- 
ture core,  and  thereby  diminishing  materially  the  great 
magnetic  resistance  which  this  space  offers  to  the  lines  of 
force.  According  to  S.  P.  Thompson,  the  magnetic  resist- 
ance of  air  may  be  20,000  times  that  of  the  iron,  which,  if 
correct,  will  show  the  great  importance  of  making  the  mag- 
netic resistance  of  this  space  as  small  as  possible  by  in- 
creasing its  area  and  diminishing  its  depth  or  thickness. 
Kapp1  states  that  the  magnetic  resistances  of  air  and  iron 
(at  low  magnetization)  are  as  1440  to  2.  Although  this  is 
much  lower  than  the  figure  given  by  Thompson,  it  shows 
that  it  is  highly  improbable  that  it  may  happen  that  the 
resistance  of  the  air  space  between  the  pole  and  the 
armature  ring  will  be  much  less  than  that  of  the  ring  itself, 

1.  Proceedings  of  Society  of  Telegraph  Engineers  and  Electricians,  Nov. 
11,  1886. 


On  Gramme  Armatures.  ^39 

except  perhaps  in  abnormally  proportioned  machines  with 
a  very  small  amount  or  a  poor  quality  of  iron  in  the  ring. 
In  such  poorly  designed  machines  the  advantage  of  interior 
pole  pieces  would  be  less. 

This  advantage  of  interior  pole  pieces,  therefore,  lies  in 
the  economy  of  the  magnetism,  but  as  this  magnetism  re- 
quires for  its  generation  only  from  2  to  10$  of  the  total 
energy  of  the  machine,  the  advantage  is  merely  in  the  re- 
duction of  this  small  fraction. 

There  is  a  disadvantage  in  having  an  interior  pole-piece 
which  in  some  cases  may  be  so  great  as  to  more  than  out- 
weigh the  advantages.  The  speed  of  the  wire  on  the  in- 
side is  necessarily  less  than  that  on  the  outside  ;  with 
small  thick  rings  this  difference  is  very  great  being  some- 
times as  great  as  40  to  50$.  In  that  case  if  the  interior 
pole-piece  merely  re-distributes  lines  of  force  by  taking 
some  from  the  outside  surface  and  leading  them  into  the 
ring  on  the  inside  surface,  it  is  evident  that  it  will  do  more 
harm  than  good,  as  many  of  the  lines  of  force  will  then  be 
cut  at  a  less  speed,  and  therefore  generate  less  potential. 

Another  disadvantage  lies  in  the  fact  that  the  two  in- 
terior pole  pieces  may,  in  the  case  of  smaller  rings,  be  quite 
near  to  the  iron  or  steel  shaft,  in  which  case  many  of  the 
lines  of  force  will  leak  directly  across  from  one  to  the  other 
through  the  shaft,  and  as  these  are  not  cut  by  the  wire  on 
the  armature,  they  are  wasted. 

One  way  of  rendering  the  inside  wire  active  is  to  place 
a  short  thick  fixed  magnet  in  the  inside  of  the  ring,  the 
poles  of  which  are  of  like  polarity  to  those  on  the  outside. 
AVith  such  a  machine  the  following  experiment  might  at 
first  appear  to  be  a  convincing  illustration  of  the  great  ad- 
vantage of  such  an  interior  magnet,  but  upon  closer  exami- 
nation the  apparent  advantage  will  be  found  to  have  been 
due  to  other  causes.  Let  the  machine  be  run  with  both 
external  and  internal  magnets  excited  and  let  its  output  be 
measured.  Then  let  another  test  be  made  but  without  ex- 
citing the  interior  magnet  ;  it  will  be  found  to  generate 


240  The  So-called  "Dead  Wire." 

very  much  less  electrical  energy,  thus  apparently  showing 
an  advantage  in  favor  of  the  interior  magnet.  But  it  will 
be  noticed  that  when  the  interior  magnet  is  not  excited 
many  of  the  lines  of  force  from  the  external  magnets  will 
pass  directly  across  the  ring  and  through  this  dead  magnet, 
which  then  acts  as  a  sort  of  magnetic  short  circuit  to  the 
poles  of  the  ring  ;  many  of  the  lines  of  force  will  then  no 
longer  be  threaded  through  the  coils  on  the  ring,  which  will 
therefore  generate  much  less  potential.  Or,  according  to 
the  other  theory,  the  wire  on  the  inside  wrill  then  cut  lines 
of  force  which  have  the  wrong  direction,  and  will  therefore 
generate  a  potential  in  the  reverse  or  opposing  direction 
similarly  to  the  case  of  the  wire  m  n  figure  2,  which,  when 
the  core  is  over-saturated,  cuts  the  wasted  lines  of  force 
a  a  which,  having  the  wrong  direction,  generate  an  oppos- 
ing electromotive  force  as  shown.  The  output  of  the 
machine  in  the  second  case  will  therefore  be  the  difference 
of  two  opposing  electromotive  forces.  The  interior  mag- 
net therefore  does  harm  when  not  excited,  the  machine 
would  in  this  case  generate  more  potential  if  the  magnet 
were  removed  altogether.  The  proper  way  to  make  such 
a  comparative  test  would  be  to  test  the  machine  first  with 
the  interior  magnet  excited  and  then  with  this  magnet  re- 
moved altogether.  Any  difference  may  then  be  safely 
attributed  to  the  interior  magnet. 

The  experiment  just  described  is  only  one  among  many 
which  show  how  easy  it  sometimes  is  to  mislead,  by  strik- 
ing results,  the  minds  of  people  who  prefer  to  trust  to  their 
own  powers  of  observation  rather  than  to  accept  the 
opinion  of  a  technical  engineer. 


APPENDIX   III. 


Explorations  of  the  Magnetic  Fields   Surrounding 
Dynamos. 

DUKING  the  Electrical  Exhibition  at  Philadelphia,  in 
1884,  the  writer  had  occasion  to  examine  the  large  Edison 
dynamo,  commonly  known  as  "  Jumbo,"  with  a  view  to 
find  what  disturbing  effect,  if  any,  the  iron  of  the  direct 
coupled  steam  engine  had  upon  the  distribution  of  the 
magnetism  of  the  two  pole  pieces,  it  being  attached  to  one 
of  them.  The  examination  was  directed  to  finding  the 
position,  polarity,  and  approximately  the  intensity,  of  mag- 
netic poles  on  different  parts  of  the  engine  and  pole  pieces. 
By  the  somewhat  strange  behavior  of  the  exploring  needle, 
the  writer's  attention  was  drawn  to  the  peculiar  distribu- 
tion of  the  magnetism  around  the  yoke  piece  and  parts  of 
the  coils  of  the  dynamo  itself  where  it  was  apparently  not 
affected  by  the  engine.  The  peculiar  results  obtained  in 
some  parts  led  to  a  similar  examination  of  other  machines 
at  the  exhibition  with  a  view  to  making  diagrams  of  the 
magnetic  fields  surrounding  the  dynamos  by  plotting  the 
direction  of  the  external  lines  of  force.  The  resulting 
diagrams  were  not  only  interesting  but  may  also  prove  to 
be  instructive  to  designers  and  builders  of  dynamos,  as 
they  show  clearly  and  conclusively  the  nature  of  the  in- 
visible field  surrounding  the  different  parts  of  the  magnets; 
and  as  a  large  part  of  this  field  is  leakage  or  wasted  mag- 
netism, they  show  how  frames  should  and  how  they  should 
not  be  constructed  with  reference  to  magnetic  distribution. 

The  tests  were  made  as  follows.  The  exploring  needle, 
which  may  be  termed  a  "  magnetoscope' "  or  indicator  of 
magnetism,  as  distinguished  from  a  "magnetometer" 
which  measures  the  quantity,  was  an  ordinary  short, 
light,  compass  needle,  strongly  magnetized  and  loosely 


242  Explorations  of  the  Magnetic  Fields 

supported  on  a  pivot.  While  the  dynamos  were  running 
and  the  magnets  fully  excited,  this  needle  was  moved 
about  in  the  space  surrounding  different  parts  of  the  mag- 
nets and  the  machine  frame.  If  the  case  containing  the 
needle  be  turned  so  that  the  needle  is  always  perpendicular 
to  its  pivot,  thus  allowing  it  to  turn  as  if  on  a  ball  and 
socket  joint,  the  needle  will  take  the  direction  of  the  lines 
of  force  in  this  space.  By  moving  it  systematically  all 
around  the  different  parts  of  the  machine,  and  by  plotting 
the  direction  of  the  needle  on  outline  drawings  of  the 
machine,  the  directions  of  the  lines  of  force  may  be  readily 
indicated.  To  explore  the  curved  paths  of  the  lines  of 
force  the  needle  may  be  held  near  to  any  particular  part 
of  the  frame  and  then  moved  continually  in  the  direction 
in  which  the  farther  end  points,  that  is,  as  the  direction  of 
the  needle  changes  the  direction  in  which  it  is  moved  must 
be  changed  so  as  always  to  conform  with  that  to  which  the 
needle  points.  The  path  which  it  describes  will  then  be 
that  of  the  particular  line  of  force.  In  this  way  the  curva- 
ture and  the  two  terminals  of  an  external  line  of  force  can 
readily  be  determined  and  plotted  with  all  due  accuracy, 
as  the  needle  is  always  a  tangent  or  short  cord  to  that 
curve,  and  where  the  needle  points  directly  to  the  iron 
when  held  closely  to  it,  it  indicates  the  point  where  the 
line  enters  or  leaves  the  iron.  By  shaking  the  needle  to 
cause  it  to  oscillate,  a  rough  approximation  can  be  obtained 
of  the  intensity  of  the  field  or  the  number  of  lines  of  force 
at  different  places.  Great  care  should  be  taken  not  to 
move  the  needle  too  fast,  as  the  lines  of  force  in  some 
machines  make  such  short  curves  that  the  needle  is  demag- 
netized and  even  reversed  in  polarity  before  it  has  time  to 
turn  on  its  pivot.  It  is  necessary,  on  account  of  this  pos- 
sible reversal  of  the  magnetism,  to  test  frequently  its 
polarity  at  one  of  the  machine  poles,  otherwise  very  queer 
and  conflicting  curves  may  be  obtained. 

The  polarity  of  a  magnet  being  usually  represented  by 
the  signs  +  and  — ,  the  former  indicating  the  north  or 


Surrounding  Dynamos.  243 

north -seeking  pole,  and  the  latter  the  south  or  south  seek- 
ing pole,  the  direction  which  the  external  lines  of  force  are 
conventionally  assumed  to  have,  is  from  -|-  to  — ,  that  is, 
as  if  they  emanated  from  the  north  pole  of  a  magnet.  If 
the  compass  needle  or  magnetoscope  has  an  arrow  head  on 
its  north  end,  the  direction  of  this  arrow  will  always  show 
the  direction  of  the  lines  of  force  of  the  field  which  is  being 
explored. 

It  will  greatly  facilitate  the  examination  and  plotting  of 
these  curves  to  remember  that  every  line  of  force  appears 
to  make  a  closed  curve  which  encircles  the  wire  through 
which  the  exciting  current  flows.  This  will  also  enable 
one  to  interpolate  the  probable  return  paths  of  the  lines  in 
the  iron  where  their  presence  cannot  be  ascertained  by 
means  of  a  needle  otherwise  than  by  their  external  mani- 
festations. All  lines  which  do  not  pass  from  the  pole 
pieces  to  the  armature  core  and  are  therefore  not  cut  by 
the  armature  coils,  are  wasted  and  represent  leakage. 

In  the  accompanying  diagrams  the  probable  return  paths 
of  a  few  of  the  lines  of  force  in  the  iron  parts  have  been 
indicated,  and  it  is  believed  they  are  correct.  The  draw- 
ings may  not  in  all  cases  represent  the  exact  proportions 
of  the  frame,  as  they  are  copied  from  mere  rough  note -book 
sketches. 

The  first  machine  examined  was  the  large  Edison  "  Jum- 
bo." Figure  1  shows  the  results  obtained,  copied  from  the 
rough  sketches.  As  is  well  known,  the  frame  of  this  ma- 
chine is  unsymmetrical,  consisting  of  two  horizontal  sets 
of  cylindrical,  parallel  magnets  in  the  upper  part,  and  one 
set  in  the  lower.  Each  set  consisting  of  four  magnets, 
there  were  altogether  eight  magnets  in  the  upper  part  and 
four  in  the  lower.  From  this  unequal  distribution  alone, 
it  might  have  been  supposed  that  either  there  would  be 
considerable  leakage  of  the  excess  of  magnetism  of  the 
upper  sets  of  magnets,  the  lower  ones  presenting  only  half 
the  cross-section  of  iron,  or  that  if  the  upper  ones  were 
only  half  saturated  in  order  not  to  over-saturate  the  lower, 


244 


Explorations  of  the  Magnetic  Fields 


there  were  more  magnets  in  the  upper  part  than  required 
to  produce  the  needed  magnetism.  The  coil  of  each  lower 
magnet  was  found  to  be  connected  in  series  with  the  two 


above  it,  thus  forming  a  series  of  three  coils  ;  these  series 
groups  were  then  connected  in  multiple  arc  with  one 
another,  forming  a  series-multiple  group  of  three  in 
series  and  four  in  multiple.  If,  therefore,  the  number  of 
turns  of  wire  was  the  same  in  each  of  the  upper  and  lower 
coils,  the  upper  half  of  the  machine  must  have  received 
twice  the  amount  of  magnetism  that  the  lower  had.  From 
the  external  lines  of  force  this  appears  in  a  measure' to 
have  been  the  case.  The  exploration  of  this  field  showed 
a  somewhat  peculiar  distribution  of  magnetism  which  it 
was  at  first  thought  might  be  due  partly  to  reversals  of 
polarity  of  the  exploring  needle,  but  repeated  examina- 
tions during  which  the  needle  was  frequently  tested  for 


Surrounding  Dynamos.  245 

polarity,  showed  that  the  results  were  quite  correct,  as 
might  have  been  shown  d  priori,  had  it  been  known  that 
the  total  ampere- turns  in  the  upper  part  were  much 
greater  than  those  in  the  lower. 

Beginning  first  with  the  upper  pole-piece  marked  N,  the 
usual  intense  leakage  was  observed  on  the  surfaces  and  at 
the  corners,  and  especially  at  the  pole-piece  projections, 
magnetically  shunting  the  armature.  Passing  next  along 
the  upper  magnet  the  first  neutral  point  marked  N  s  was 
found  beyond  the  middle,  the  lines  of  force  on  the  other 
side  of  it,  entering  instead  of  emanating  from  the  cores,  as 
before.  Passing  next  to  the  upper  end  of  the  yoke-piece 
a  strong  south  pole  was  found.  The  lower  one  of  the 
upper  magnets  was  found  to  be  similar  on  its  lower  side, 
to  the  upper  side  of  the  upper  magnet,  only  that  the  neu- 
tral point  was  nearer  the  yoke-piece  than  before, — neutral 
point  being  understood  to  mean  a  point  where  the  two 
poles  are  exactly  equal  in  intensity  and  where,  therefore,  the 
external  lines  of  force  are  parallel  to  the  cores.  In  a  nor- 
mal, straight  bar  magnet,  the  neutral  point  will  always  be 
in  the  middle,  and  might  be  termed  the  magnetic  middle 
of  a  magnet,  which  in  a  normally  proportioned  bar  magnet 
should  coincide  with  its  linear,  axial  middle. 

In  the  space  between  these  two  upper  parallel  magnets, 
it  was  found  that  the  lines  of  force  were  quite  straight 
along  their  whole  length,  and  parallel  to  the  magnets,  but 
in  the  opposite  direction  to  their  course  in  the  cores.  In 
other  words,  these  two  like  magnets  were  neutral  along 
their  whole  length  on  the  sides  facing  each  other.  This 
might  be  considered  as  another  proof  that  in  two  such 
parallel  like  coils  ending  in  the  same  pole-piece,  those 
parts  of  the  coils  which  are  nearest  each  other  neutralize 
one  another  as  far  as  the  useful  magnetism  of  the  pole- 
piece  is  concerned  ;  it  will  be  noticed  that  these  parallel 
lines  of  force  when  continued,  as  shown,  through  the  cores, 
encircle  those  portions  of  the  coils. 

Passing  next  down  the  yoke-piece  it  was  found  that 
instead  of  being  neutral  in  the  middle,  as  usual,  with  poles 


246  Explorations  of  the  Magnetic  Fields 

at  each  end,  it  was  a  strong  south  pole  along  its  whole 
length  with  no  signs  of  a  neutral  point  at  any  place.  This 
is  no  doubt  accounted  for  by  the  great  excess  of  magnet- 
ism of  the  upper  magnets  over  that  in  the  lower. 

The  south  or  lower  pole-piece  was  quite  similar  to  the 
north,  showing  the  usual  intense  leakage  on  all  sides  and 
corners  and  especially  at  the  pole-piece  projections. 

Passing  finally  to  the  lowest  magnet  a  very  peculiar  re- 
sult was  obtained,  which  was  at  first  discredited,  but  upon 
repeated  and  careful  exploring  it  was  found  to  be  verified. 
A  short  distance  from  the  right  hand  end  there  was  a 
very  intense  north  pole,  the  lines  of  force  being  at  that 
point  quite  perpendicular  to  the  coil  for  a  short  distance, 
as  shown,  and  afterwards  branching  out  to  the  right  and 
left,  finally  entering  the  iron  again.  Between  this  point 
and  the  yoke-piece,  there  was,  of  course,  a  neutral  point, 
marked  N  s,  and  on  the  left  hand  side,  also  quite  near  to 
it,  another  neutral  point  of  opposite  polarity,  marked  s  N. 
These  two  neutral  points  situated  so  very  near  a  strong 
pole,  created  a  very  curious  field,  the  most  interesting  part 
of  which  is  that  lying  above  them  where  the  lines  come  to- 
gether with  those  of  the  magnet  above  it.  At  this  point, 
a,  marked  by  a  dot,  it  will  be  noticed  that  the  lines  which 
come  together  make  four  sharp  turns  as  though  there  was 
something  at  this  point  which  did  not  permit  the  lines  to 
pass  through  but  which  reflected  them  somewhat  as  rays 
of  light  are  reflected  from  a  mirror.  A  needle  held  at 
this  point  acts  as  though  it  were  uncertain  in  which  direc- 
tion to  point,  and  if  moved  about  this  point  even  for  very 
short  distances,  it  will  have  very  decided  but  quite  differ- 
ent directions,  as  will  be  seen  from  the  arrows  which  show 
the  direction  of  the  needle  when  in  the  respective  lines 
of  force.  If  a  very  short  needle  could  be  placed  exactly 
at  this  point  it  would  act  similarly  to  a  body  balanced  in 
unstable  equilibrium,  the  slightest  movement  from  this 
position  causing  the  needle  to  deflect  in  a  certain  definite 
direction.  Such  points  might  therefore  be  termed  "points 
of  unstable  magnetism,"  or  "points  of  magnetic  deflection 


Surrounding  Dynamos.  247 

having  four  poles,"  as  there  are  four  general  directions  of 
the  lines  of  force  in  the  vicinity  of  this  point.  Their 
characteristic  peculiarity  is  that  the  needle  is  very  apt  to 
have  its  polarity  reversed  when  moved  through  them,  the 
explanation  of  which  is,  no  doubt,  that  the  lines  of  force 
make  such  abrupt  curves  that  their  direction  through  the 
needle  is  reversed  before  it  has  time  to  turn  on  its  pivot. 

In  a  few  places  the  probable  courses  of  the  lines  of  force 
through  the  iron  itself,  have  been  indicated.  It  is  not 
possible  to  determine  this  definitely  with  the  aid  of  a 
needle  only,  but  from  the  nature  of  the  field  surrounding 
the  iron  and  from  the  general  laws  of  lines  of  force,  it  is 
very  likely  that  the  courses  indicated  are  correct. 

From  the  results  obtained  in  the  exploration  of  the  field 
of  this  dynamo,  the  following  conclusions  may  be  drawn  : 
Two  similar  parallel  magnets,  like  those  in  the  upper  part, 
ending  in  the  same  pole  and  yoke-piece,  are  not  to  be  rec- 
ommended as  there  is  considerable  leakage  between  them. 
When  the  general  form  of  the  magnets  for  a  dynamo  is 
that  of  a  simple  U  magnet,  it  should  if  possible  be  sym- 
metrical magnetically  as  in  an  ordinary  horse-shoe  magnet. 
The  magnetism  of  that  part  of  the  lower  magnet  between 
the  letter  N  and  the  yoke-piece,  is  in  the  opposite  direc- 
tion to  that  generated  by  the  ampere-turns  on  this  part, 
from  which  it  appears  that  the  magnetism  from  a  certain 
number  of  ampere-turns  on  other  parts  of  the  magnet  is 
consumed  in  first  neutralizing  that  in  this  part  and  then 
reversing  it  ;  it  appears  from  the  strong  north  pole  on  the 
lower  magnet  that  the  economy  in  the  magnetism  from 
the  ampere-turns  is  poor,  the  reason  being  apparently  in 
the  unsymmetrical  distribution  of  the  iron  and  coils.  As 
most  of  the  lines  of  force  shown  in  the  figure  do  not  pass 
through  the  armature,  they  represent  leakage  or  wasted 
magnetism. 

Figure  2  shows  one-half  of  a  normally  proportioned 
Weston  machine,  the  general  type  of  frame  being  used  also 
by  numerous  other  makers.  The  field  surrounding  this 


248 


Explorations  of  the  Magnetic  Fields 


frame  appears  to  be  quite  symmetrical  above  and  below, 
which  in  fact  would  naturally  follow  from  the  symmetri- 


cal distribution  of  the  iron  and  the  magnetizing  coils.  The 
two  neutral  points  on  the  upper  and  lower  surfaces  of  the 
same  magnet  were  not  directly  above  each  other,  that  on 
the  outside  surface  being  nearer  the  yoke-piece.  Both  the 
upper  and  lower  coils  showed  strong  poles  at  each  end. 
The  yoke-piece  has  strong  poles  at  its  ends  and  neutral 
points  in  the  middle.  No  portions  of  the  coils  have  re- 
versed magnetism  in  them,  which  is  no  doubt  due  to  the 
symmetrical  distribution  of  the  magnetism.  A  point  of 
"  unstable  magnetism  "  or  of  "  magnetic  deflection  "  was 
found  in  this  machine  also,  differing  from  the  one  observed 
in  the  Edison  machine  in  the  fact  that  the  lines  of  force 
around  it  were  symmetrically  distributed.  The  gene- 


Surrounding  Dynamos.  249 

ral  character  of  the  field  at  such  a  point  is  more  clearly 
shown  in  this  machine.  The  upper  and  lower  lines  of  force 
having  opposite  directions  or  opposite  polarity,  will,  ac- 
cording to  the  laws  of  lines  of  force,  attract  each  other, 
forming  sharp  curves  at  their  nearest  point ;  the  same  is 
true  of  the  lines  on  the  right  and  left  hand  sides  of  this 
point,  thus  forming  the  four  curves  shown.  The  field  at 
this  point  is  similar  to  that  produced  by  four  bar  magnets 
placed  in  the  position  shown,  the  opposite  ones  having  like 
poles  toward  each  other.  It  will  be  noticed  that  this  point 
is  bounded  by  four  neutral  points  (one  of  which  is  in  the 
armature)  alternating  with  four  poles,  the  successive  neu- 
tral points  and  poles  being  alternately  of  opposite  polarity, 
a  neutral  point  being  considered  to  be  a  point  in  the  mag- 
netic circuit  around  which  the  external  lines  of  force  are 
parallel  to  the  axis,  as  described  before.  This  is  also  true 
of  that  point  in  the  Edison  machine,  only  that  the  distri- 
bution is  not  as  regular  as  in  the  Weston.  In  both  these 
machines,  and  probably  in  all  others  having  such  \J  mag- 
nets with  two  coils,  the  complete  iron  circuit  of  the  useful 
lines  of  force,  beginning  at  one  pole-piece  thence  succes- 
sively through  one  magnet,  the  yoke-piece,  the  other  mag- 
net, the  other  pole-piece,  and  finally  returning  through  the 
armature,  includes  four  alternately  opposite  poles,  separa- 
ted by  four  neutral  points  of  alternately  opposite  polarity, 
the  polarity  of  the  neutral  points  being  indicated  by  N  s 
or  s  N  respectively.  If  the  entire  |J  magnet  were  made 
in  the  form  of  a  circle  wound  over  its  whole  length  with 
coils,  like  a  Faraday  induction  coil,  and  cut  at  one  point 
for  inserting  the  armature  as  in  the  Sir  Wm.  Thomson 
and  the  Mather  machines,  it  is  possible  that  this  succession 
of  poles  and,  therefore,  also  the  point  of  magnetic  deflection, 
would  disappear. 

Figure  3  shows  a  Sprague  motor  in  which  two  neutral 
points  were  found  near  the  far  ends  of  two  alternate  coils, 
Avhile  none  appeared  to  exist  on  the  other  coils.  Unfavor- 
able circumstances  prevented  the  further  examination  of 


250 


Explorations  of  the  Magnetic  Fields 


this  machine;  but  it  is  quite  probable  that  it  would  prove  to 
be  similar  to  the  Edison  machine  in  figure  1,  as  there  ap- 
peared to  be  two  series-wound  coils  on  two  alternate  mag- 
nets, and  two  shunt  coils  on  the  other  two,  which,  unless 


the  ampere-turns  were  equal  in  both,  would  cause  unequal 
distribution  of  magnetism  ;  the  location  of  the  two  neutral 
points,  as  shown,  indicates  that  this  may  have  been  the 
case  with  this  particular  machine  while  it  was  being  exam- 
ined. The  unsym metrical  distribution  is  probably  inten- 
tional in  this  machine,  to  accomplish  certain  results. 

Figure  4  shows  the  results  obtained  from  a  Van  Depoele 
machine,  the  frame  of  which  is  of  an  entirely  different 
class  from  those  already  shown.  Only  one-half  is  shown 
here  ;  the  coils  surround  the  large  massive  cores  which  ter- 
minate each  in  one  pole-piece.  On  the  upper  and  lower 
side  of  the  frame  are  two  flat  pieces  which  were  presum- 
ably intended  for  yoke  pieces  to  magnetically  connect  the 
south  pole  at  the  right  hand  end  of  the  one  core  with  the 
north  pole  at  the  other  end  of  the  other  core.  The  ex- 
ploration of  the  field,  however,  indicates  that  it  performs 
the  entirely  different  and  very  objectionable  function  of 
magnetically  shunting  the  armature  by  inducing  the  other- 


Surrounding  Dynamos.  251 

wise  useful  lines  of  force  to  leave  the  edge  of  the  north 
pole-piece  and  return  through  this  intended  yoke-piece  to 
the  south  or  right  hand  end  of  the  magnet,  thus  acting  as 
a  superfluous  and  objectionable  yoke-piece  to  the  two  ends 
of  tlie  same  magnet;  the  lines  of  force  which  should  pass 
through  the  armature  are  therefore  shunted  off  to  one  side. 
Furthermore,  that  portion  of  this  flat  piece  which  lies  di- 
rectly above  the  two  pole -piece  projections  marked  s  and 
N,  appears  to  short  circuit,  magnetically,  these  two  pole 
pieces,  or  in  other  words,  it  magnetically  shunts  the  arma- 
ture, as  a  portion  of  the  lines  of  force  which  should  pass 
through  the  armature  are  led  off  around  it  and  are,  there- 
fore, wasted.  If  this  is  as  it  appears  to  be,  it  might  be 
better  to  omit  these  pieces  altogether  or  to  make  them  of 
some  diamagnetic  material. 

The  strongest  leakage  appears  at  the  two  pole-piece  pro- 
jections, the  leakage  being  into  the  intended  yoke-piece. 
This  leakage  extends  along  the  side  of  the  coil,  nearly  to 
the  neutral  point  N  s,  which  is  near  the  farther  end.  The 
yoke-piece  also  exhibits  a  neutral  point  nearly  above  the 
one  in  the  magnet  and  of  the  same  polarity.  As  might  be 
supposed,  there  is  very  intense  leakage  from  the  ends  of 
the  machine,  the  right  hand  end  in  the  figure  being  a  very 
strong  south  pole.  But  the  leakage  at  this  place  does  not 
necessarily  represent  wasted  magnetism,  as  most  of  the  lines 
of  force  which  escape  here  no  doubt  pass  through  the 
armature  after  passing  through  the  core  of  the  magnet, 
and  are  therefore  not  wasted;  their  return  course,  however, 
is  to  a  great  extent  through  the  air,  thus  making  open  cir- 
cuit magnets  as  distinguished  from  closed  circuit  or  iron- 
clad magnets  in  which  the  lines  pass  through  iron  along 
their  whole  course,  and  as  the  magnetic  resistance  of  air  is 
much  greater  than  that  of  iron  (according  to  Kapp  720 
times  that  of  iron)  the  magnetism  must  be  weakened  con- 
siderably in  passing  through  so  much  air  in  its  return 
circuit. 

A  peculiar  field  was  noticed  outside  of  the  yoke-piece 


252 


Explorations  of  the  Magnetic  Fields 


almost  directly  above  the  middle  of  the  machine.  In 
moving  the  exploring  needle  horizontally  at  this  place  it  sud- 
denly made  an  abrupt  turn  of  about  90°  and  over,  indicat- 


ing a  sharp  turn  of  the  lines  of  force  at  this  point.  Further 
examinations  indicated  that  the  lines  of  force  had  the  di- 
rection as  shown  at  this  point,  which  can  readily  be  ex- 
plained on  the  assumption  that  there  exists  in  the  iron  of 
the  yoke-piece  directly  below  these  sharp  curves,  a  point 
of  deflection  of  magnetism  or  point  of  unstable  magnet- 
ism as  it  was  termed  above,  which  has  been  indicated  in 
the  figure  by  four  short  curves.  Not  only  do  the  lines  of 
force  all  around  this  point  indicate  its  presence,  but  in  a 
very  similar  machine  (figure  7)  the  same  point  was  shown 
to  exist  in  nearly  the  same  relative  position,  being  in  this 
case  in  the  air  instead  of  in  the  iron,  and  its  existence 
could  therefore  be  determined  definitely.  If  lines  of  force 


Surrounding  Dynamos.  253 

form  closed  circuits  in  themselves  it  is  probable  that  those 
which  are  deflected  downward  to  this  point  without  enter- 
ing the  iron,  are  the  return  circuits  of  those  which  leak 
out  at  the  two  ends  of  the  machine. 

The  field  around  the  Thomson-Houston  machine,  shown 
in  figures  5,  6  and  7,  proved  to  be  one  of  the  most  interest- 
ing, some  of  the  curves  being  of  a  very  peculiar  nature. 
The  magnetism  was  so  intense  in  several  places  and  the 
lines  made  such  sharp  turns  that  the  exploring  needle  was 
repeatedly  reversed  in  polarity  during  the  examination,  thus 
giving  absurd  results.  It  was  not  until  the  needle  was  re- 
peatedly tested  for  polarity,  and  all  results  obtained  with 
a  reversed  needle  discarded,  that  any  conclusive  results 
could  be  obtained.  The  test  was  furthermore  rendered 
difficult  at  first  by  the  fact  that  the  direction  of  the  lines 
of  force  could  not  be  tested  in  one  plane  only  as  in  the 
other  machines,  as  they  curved  around  in  different  planes  in 
such  a  way  as  to  make  it  difficult  to  represent  their  courses 
on  the  plane  of  the  drawing.  In  order  to  be  sure  that  the 
results  were  correct  the  writer  recently  repeated  the  tests 
very  carefully  on  a  new  30-light  arc  machine,  and  obtained 
the  results  shown  in  the  figures,  which  are  believed  to  be 
substantially  correct. 

Figure  5  shows  more  than  half  of  the  vertical  cross-section 
through  the  centre,  taken  perpendicularly  to  the  shaft. 
The  coils  are  wound  around  the  large,  hollow  cast-iron  cores 
which  terminate  at  the  armature  and  at  the  ends  of  the  ma- 
chine, one  of  which  is  shown  complete.  There  is  a  round 
opening  in  the  thin  shell  which  embraces  the  right  and  left 
hand  sides  of  the  armature.  The  intended  yoke-piece  consists 
of  a  series  of  horizontal,  round,  wrought-iron  bars,  two  of 
which  are  shown,  one  above  and  the  other  below  the  arma- 
ture. It  is,  therefore,  similar  in  general  outline  to  the  Van 
Depoele  machine  in  figure  4,  and  the  results  should  there- 
fore agree  to  a  certain  extent  with  those  shown  in  figure 
4,  which  was  found  to  be  the  case.  As  in  the  Van  De- 
poele, there  was  a  very  strong  leakage  from  the  magnets 


254 


Explorations  of  the  Magnetic  Fields 


to  the  yoke-piece,  especially  at  and  near  the  ends  of  the 
pole  pieces,  showing  that  many  lines  of  force  were  mag- 
netically shunted  around  the  armature,  and  were  therefore 
wasted.  A  neutral  point  was  found  near  the  right  hand 


Fig. 5 


end  of  the  core  as  shown.  Immediately  above  the  bars  of 
the  yoke-piece  a  strong  field  existed  which  showed,  the 
same  polarity  as  that  between  the  yoke  and  pole-piece,  as 
indicated  by  the  arrows,  showing  that  these  yoke-pieces  had 
two  strong  poles  one  above  each  of  the  pole  pieces,  but  of 
opposite  polarity  to  the  latter.  Immediately  above  this 


Surrounding  Dynamos.  255 

field  and  represented  by  an  open  space,  the  needle  exhibited 
that  peculiar  state  of  unstability,  indicating  a  point  of  de- 
flection, while  above  this  space  it  again  had  a  definite  and 
decided  direction.  In  this  space,  left  blank  on  the  figure, 
the  lines  were  found  to  be  perpendicular  to  the  plane  of 
the  drawing,  as  is  shown  more  clearly  in  figure  6,  which  is 


an  enlarged  vertical  cross-section  taken  through  tbe  north 
pole-piece  perpendicular  to  the  bars  of  the  yoke-piece.  As 
shown  in  this  figure  the  lines  passed  from  the  pole-piece 
into  the  yoke-piece  on  all  sides,  while  the  others  from  the 
pole-piece  pass  out  into  the  air  between  the  bars,  thus 
forming  above  the  bars  at  about  the  relative  distance 
shown,  a  point  of  deflection  having  four  poles,  similar  to 
that  in  the  Edison  and  Weston  machines. 

Referring  again  to  figure  5  a  peculiar  deflection  of  the 
lines  was  noticed  almost  directly  above  the  south  pole- 
piece,  which  was  somewhat  similar  to  that  observed  in  the 
Van  Depoele  machine.  It  is  possible  that  this  was  due  to 
another  point  of  deflection  existing  in  the  iron,  as  indicated 
in  figure  5,  with  four  or  possibly  six  poles,  but  these  curves 
in  the  iron  itself  are  probable  conjectures  only.  A  similar 
deflection  was  noticed  a  few  inches  to  the  left  of  this 
point. 

In  moving  the   exploring  needle   vertically  above  the 


256 


Explorations  of  the  Magnetic  Fields 


north  pole-piece,  it  was  found  to  turn  abruptly  through 
almost  180°  a  few  inches  above  the  yoke-piece,  as  shown 
by  the  arrows,  while  nearly  above  this  there  was  a  deflec- 
tion similar  to  that  in  figure  4.  This  was  found  to  be  due 
to  a  point  of  deflection  existing  between  the  bars  but 
above  them,  and  is  shown  more  clearly  in  figure  7,  which 


represents  a  portion  of  a  section  like  figure  5,  but  between 
the  bars.  This  point  was  clearly  indicated  by  the  needle, 
and  assists  in  connection  with  figures  5  and  6,  in  explain- 
ing some  of  the  peculiar  curves  noticed  above  that  part  of 
the  yoke-piece  which  lies  above  the  pole  pieces  in  figure  5. 

At  the  right  hand  end  of  the  core  the  usual  strong  pole 
was  found.  A  peculiar  curving  of  the  lines  was  noticed 
at  5,  where  the  rounded  projections  of  the  wrought-iron 
bars  passed  through  the  flanges.  A  possible  explanation 
of  this  is,  that  the  cast-iron  flange  is  over-saturated  to  a 
greater  extent  than  this  projection  of  the  wrought-iron 
bar,  which,  therefore,  attracts  the  lines. 

Following  the  surface  of  the  core  to  the  inside  or  hollow 
portion,  a  weak  south  pole  was  found  to  exist  on  the  cylin- 
drical surface,  while  on  the  interior  flange  embracing  the 
armature,  the  south  pole  was  found  to  be  very  strong,  the 
lines  from  it  passing  out  through  the  open  end.  By  placing 


Surrounding  Dynamos.  257 

a  small  needle  very  near  the  revolving  armature  and  in 
the  opening  in  the  iron,  it  was  found  that  many  of  the 
lines  from  this  south  pole  entered  the  armature.  From 
the  general  principles  underlying  all  other  dynamos,  it 
would  be  supposed  that  the  polarity  of  the  pole-piece  em- 
bracing the  right  hand  half  of  the  armature,  was  the  same 
in  all  parts,  but  it  appears  from  the  external  lines  of  force 
which  could  be  examined,  that  this  is  not  the  case  with 
this  machine  as  there  is  a  strong  north  and  a  strong  south 
pole  on  different  parts  of  the  same  pole-piece.  Careful 
and  repeated  examinations  with  the  exploring  needle  on 
the  same  and  on  different  machines  clearly  shows  this 
peculiar  disposition  of  poles  to  exist. 

Near  the  opening  through  the  centre  of  the  core,  was 
another  point  of  deflection  with  four  poles,  which  in  some 
machines  was  found  to  be  very  near  the  armature,  as 
shown  on  the  left  hand  side,  while  in  others  it  was  located 
several  inches  from  the  armature  as  shown  on  the  right 
hand  side.  The  lines  of  force  at  this  point  were  very'in- 
tense,  and  their  curved  directions  were  very  decided,  for 
this  reason  it  was  found  that  a  very  short  needle  had  to 
be  used  and  that  it  had  to  be  moved  about  very  slowly  to 
prevent  it  from  having  its  polarity  reversed  before  it 
could  respond  to  the  change  of  direction.  By  moving  the 
needle  rapidly  from  the  armature  through  this  point  of 
deflection,  its  polarity  can  readily  be  reversed,  and  it  will 
then  point  in  the  same  direction  as  it  did  before  passing 
the  point,  thus  making  it  appear  as  if  there  was  no  point 
of  deflection.  Moving  the  needle  toward  the  armature 
will  not  reverse  it  so  readily  as  these  curves  are  less 
abrupt. 

The  lines  of  force  at  this  point  appear  to  oscillate  very 
rapidly,  which  is  probably  due  to  the  pulsating  character  oft 
the  magnetizing  current.  If,  therefore,  a  small  induction 
coil  in  the  form  of  a  simple  coil  of  copper  wire,  be  held 
concentric  with  this  opening  in  the  core  and  near  to  it,  an 
alternating  current  would  no  doubt  be  induced  in  it.  A 


258  Explorations  of  the  Magnetic  Fields 

telephone  in  series  with  this  coil  will  indicate  the  presence 
of  such  alternating  currents  by  a  buzzing  sound,  the  note 
emitted  being  apparently  dependent  on  the  speed  and 
number  of  coils  of  the  armature. 

It  is  quite  probable  that  the  curves  shown  in  all  these 
figures  would  change  very  much  with  different  degrees  of 
saturation.  A  careful  study  of  the  curves  of  eacli  machine 
at  regularly  increasing  degrees  of  saturation,  might  lead 
to  a  determination  of  the  best  proportions  and  shapes  of 
the  iron  parts. 

Figures  4  and  5  are  good  illustrations  of  how  not  to 
proportion  the  parts  when  the  best  economy  of  magnetiza- 
tion is  desired.  At  the  same  time  it  must  not  be  forgot- 
ten that  the  energy  used  to  excite  the  magnetism  is  for 
the  best  machines  only  a  few  per  cent,  of  the  whole  out- 
put, and  that,  therefore,  any  improvements  in  the  frame 
that  would  economize  magnetism  would  only  lessen  this 
already  small  fraction,  and  possibly  in  some  cases  econo- 
mize material.  Other  conditions  may  be  of  so  much  more 
importance  as  to  make  an  increase  in  the  economy  of  mag- 
netism of  little  significance  in  comparison.  The  fact  that 
the  Thomson-Houston  machines  are  used  in  such  large 
numbers,  shows  that  great  economy  of  magnetism  is  not 
one  of  the  most  important  points  about  a  machine. 

It  is  often  represented  as  a  good  feature  of  a  machine 
that  its  magnets  are  very  powerful  and  will  hold  very 
heavy  weights  ;  a  little  consideration,  however,  will  show 
that  such  external  magnetism,  especially  when  at  the  pole 
pieces,  represents  leakage  and  waste,  and,  therefore,  is  an 
objectionable  feature.  It  would  be  much  more  creditable 
to  a  designer  of  a  machine  to  be  able  to  show  that  when 
running  with  its  full  load  the  magnets  will  not  even 
attract  a  small  bunch  of  keys,  all  the  magnetism  -being 
diverted  into  the  armature  where  it  is  wanted. 

From  the  nature  of  the  fields  surrounding  a  dynamo,  as 
shown  in  the  figures,  the  following  general  deductions 
may  be  made,  That  machines  exhibit  the  following  mag- 


Surrounding  Dynamos.  259 

netic  properties  :  Free  north  and  south  poles,  neutral  points 
of  two  different  polarities,  and  points  of  deflection  ;  the 
latter  exist  in  the  external  fields  but  may  possibly  exist 
also  in  the  iron  itself  ;  they  generally  appear  as  four  curva- 
tures of  the  lines  of  force  towards  and  receding  from  one 
point,  and  are  in  that  case  enclosed  by  four  free  poles  and 
four  neutral  points  of  alternately  opposite  polarity.  That 
the  lines  of  force  in  and  around  a  magnet  appear  to  have 
the  following  properties  :  Like  lines  repe!3  unlike  lines  at- 
tract each  other  ;  they  never  intersect  ;  they  appear  to 
form  circuits  or  paths  closed  in  themselves  and  surrounding 
the  current  generating  them  ;  when  not  prevented  by 
other  forces  they  take  the  shortest  distance  between  the 
iron  parts.  The  illustrations  of  these  statements  in  the 
special  cases  shown,  is  not  offered  as  a  proof  of  their  cor- 
rectness in  all  cases,  they  are  given  here  only  as  general 
guides  in  exploring  and  plotting  such  fields. 


APPENDIX  IV. 


Systems  of  Cylinder -Armature  Windings* 

THE  cylindrical  surface  of  the  armature  is  for  convenience 
in  winding  divided  into  sections  or  fields.  In  the  accompany- 
ing illustrations  the  number  of  sections  (denoted  by  heavy 
lines)  has  been  made  equal  to  the  number  of  coils,  in  which 
case  each  coil  must  fill  one  half  of  each  of  two  opposite  sections. 

In  figure  1,  start- 
ing at  the  upper 
commutator  bar,  the 
wire  is  wound  into 
the  first  half  of  the 
first  section,  thence 
back  through  one  of 
the  opposite  half  sec- 
tions b,  c,  d,  e,  or  /, 
and  thence  to  the  next 
commutator  bar,com- 
pleting  one  coil.  By 
winding  into  the  dia- 
metrically opposite 
half  sections  a  and  d, 
it  will  be  the  Siemens 
winding,  shown  com- 
pleted in  figure  2. 
By  winding  into  a 
and  e  it  will  be  the  Froehlich  winding,  figure  3 ;  a  and  c  will 
be  the  Breguet  winding;  a  and  6,  or  a  and  /will  both  give 
such  irregular  windings  that  they  should  not  be  used.  The 
accompanying  diagrams  show  the  completed  windings,  and 
the  five  principal  modifications  of  the  same. 

*  Abstract  from  an  article  on  Cylinder- Armatures  in  the  Electrician 
and  Electrical  Engineer,  Vol.  4,  1885,  p.  423,  and  Vol.  5,  1886,  p.  84. 

(261) 


Fig.  1. 


262  Systems  of  Cylinder- Armature  Windings. 

In  these  figures  the  windings  are  shown  diagrammatically 
as  seen  from  the  commutator  end  of  the  armature,  the  dotted 
lines  representing  the  winding  across  the  other  (pulley)  end 
of  the  armature.  The  two  halves  of  the  armature  being  in 
multiple  arc,  those  sets  of  coils  which  form  one  half  are  shown 
by  white  coil  spaces,  and  will  be  termed  light  coils,  while 
those  forming  the  other  half  are  shown  by  section  lined  coil 
spaces,  and  will  be  termed  dark  coils.  The  coil  spaces  which 
are  marked  with  crossed  Hues  represent  the  coils  which  are 
short-circuited  by  the  brushes.  The  arrow  heads  on  the  lines 
show  the  direction  in  which  the  current  will  flow  in  those 
wires  when  the  armature  revolves  to  the  left,  as  shown  by  the 
arrow  on  the  outside,  and  when  the  polarity  and  the  position 
of  the  pole-pieces  is  as  shown  by  the  letters  N  and  S.  The 
commutator  connections  of  the  dead  coils  which  are  short- 
circuited  by  the  brushes  are  indicated  by  the  letters  0  0  and 
have  no  arrow  heads.  To  avoid  too  many  lines  the  brushes 
are  drawn  outside  of  the  armature,  their  proper  place  on  the 
commutator  being  shown  by  a  small  dot.  The  diagrams  thus 
show  at  a  glance  whether  the  winding  is  symmetrical,  whether 
the  light  and  dark  coils  of  the  two  halves  of  the  armature  are 
symmetrically  situated  in  the  fields,  in  what  part  of  the  field 
the  dead  coils  lie,  the  position  and  polarity  of  the  brushes,  etc. 

The  windings  have  all  been  started  alike  by  connecting  the 
beginning  of  the  first  coil  to  the  nearest  commutator  bar,  this 
being  the  usual  way.  They  may,  however,  be  started  from  any 
other  commutator  bar,  without  changing  the  system  of  winding, 
by  merely  shifting  the  brush  line  or  the  relative  position  of 
the  commutator  bars  (and  brushes)  with  their  respective  coils. 

The  Siemens  or  Hefner-Alteneck  winding,  figure  2,  is  irreg- 
ular after  one  half  of  the  armature  is  wound  (between  the  two 
lower  white  sections).  The  coils  have  not  symmetrical  positions 
in  the  fields.  At  least  one  of  the  short-circuited  coils,  being  in 
opposite  fields,  is  not  dead.  All  the  coils  cross  each  other  on 
the  ends  of  the  armature,  making  a  bulky  "  head."  It  appears 
to  be  no  longer  used,  presumably  owing  to  these  objections. 

The  Froehlich  winding,  figure  3,  is  quite  regular.  The  dark 
and  light  coils  have  symmetrical  positions  in  the  fields.  Each 


Systems  of  Cylinder- Armature  Windings.  263 

short-circuited  coil  lies  in  one  field  and  is  therefore  more 
nearly  dead ;  the  injurious  currents  in  them  will  therefore  be 
less  than  in  the  Siemens.  Every  two  coils  being  parallel 
there  will  be  much  fewer  crossings  of  wires  at  the  ends,  which 
is  an  additional  advantage.  In  this  system  special  care  should 
be  taken  that  the  coils  are  wound  in  their  proper  spaces,  as 
there  are  always  two,  for  instance  i  and  A,  which  are  opposite 
to  one,  g,  i  is  the  right  one  and  h  the  wrong  one. 

The  Breguet  winding,  figure  4,  is  practically  identical  with 
the  Froehlich ;  the  only  difference  is  in  the  relative  positions 
of  the  commutator  bars  (brush  line)  and  their  respective  coils. 

The  Edison  system,  figure  5,  is  the  Siemens  applied  to  an 
odd  number  of  coils.  It  is  quite  regular  and  the  light  and 
dark  coils  are  symmetrically  situated.  Only  one  coil  is  short- 
circuited,  instead  of  two,  as  in  the  other  systems.  All  the  coils 
cross  each  other  as  in  the  Siemens.  It  is  simpler  to  wind  than 
either  of  the  others.  Using  an  odd  number  of  coils,  and  wind- 
ing from  a  to  c,  figure  5,  instead  of  a  to  b,  will  give  a  very 
irregular  winding,  while  from  a  to  d  will  give  a  regular  one. 

Instead  of  winding  two  neighboring  coils  next  to  one  an- 
other, as  in  figures  2  and  3,  they  may  be  wound  over  each 
other,  thus  giving  the  double  Siemens  and  double  Froehlich 
windings  shown  in  figures  6  and  7.  The  advantage  is  that 
they  are  easier  to  wind.  They  have  the  same  characteristics 
as  in  the  first  forms,  with  the  additional  disadvantage  that  the 
outer  and  inner  coils  have  different  mean  lengths  and  speeds. 
The  Edison  system  may  also  be  wound  double,  but  it  then 
appears  to  lose  its  characteristic  regularity. 

In  the  Weston  winding,  figure  8,  and  the  Hering  winding, 
figure  9,  each  coil  is  split  into  two  halves,  one  half  being 
wound  in  the  lower  layers  and  one  half  in  the  upper  layers. 
This  makes  the  mean  length  and  speed  of  the  coils  the  same, 
and  therefore  overcomes  the  objection  to  the  double  windings. 
They  are  somewhat  more  troublesome  to  wind.  The  Weston 
is  based  on  the  Siemens,  and  has  all  the  characteristics  of  that 
system,  the  objectionable  effect  of  the  irregularity  being,  how- 
ever, greatly  reduced.  The  Hering  winding  is  based  on  the 
Froehlich,  and  has  all  the  characteristics  of  that  system. 


264  Systems  of  Cylinder- Armature  Windings. 


Systems  of  Cylinder- Armature  Windings.  265 


266  /Systems  of  Cylinder- Armature  Windings. 


Systems  of  Cylinder- Armature   Windings. 


267 


APPENDIX  V. 


Equivalents  of  Units  of  Measurement. 

THE  numbers  in  these  tables  have  all  been  calculated  from 
the  following  set  of  fundamental  standard  equivalents,  viz. : 
The  values  of — 

the  meter  in  inches, 

the  gram  in  grains, 

the  avoirdupois  pound  in  grains,  . 

the  troy  (apothecary)  pound  in  grains, 

the  gallon  in  cubic  inches, 

the  atmospheric  pressure  in  millimeters  of  mercury, 

the  specific  gravity  of  mercury, 

Joules  equivalent  772, 

acceleration  of  gravity  981, 

legal  ohm  in  mercury  units, 

B.  A.  unit  of  resistance  in  legal  ohms. 

The  numerical  values  which  were  used  for  this  set  of  funda- 
mental equivalents  are  those  given  in  their  respective  places  in 
the  tables.  For  any  subsequent,  more  accurate,  determination 
of  these  fundamental  values  the  corresponding  corrections  of 
their  derived  equivalents  in  these  tables  can  readily  be  made 
by  simple  proportion.  For  this  reason  the  derived  equiva- 
lents have  in  many  cases  been  carried  to  more  decimals  than 
would  otherwise  be  reasonable.  The  values  for  the  weights 
and  volumes  of  water  are  based  on  the  metric  system. 

In  almost  all  cases  the  reciprocal  of  each  equivalent  is  also 
given  (in  its  proper  place),  thus  enabling  all  calculations  to 
be  made  by  multiplication  instead  of  division. 

The  "Approximate  Values"  have  been  given  in  the  smallest 
number  of  digits,  generally  only  one  or  two  besides  the  digits 
0  and  1.  Almost  all  of  these  approximate  values  are  within 
3%  of  the  correct  value,  and  many  of  them  within  2%. 

All  the  equivalents  have  been  calculated  by  two  independent 
methods,  and  it  is  believed  that  there  are  no  errors 

269 


EQUIVALENTS  OF  UNITS  OF  MEASUREMENT. 

CARL  BERING. 


UNITS. 
(In  order  of  size.) 

EQUIVALENTS. 

APPROXIMATE 

VALUES)  WITHIN  A 
FEW  PER  CENT. 

LOGARITHMS. 

i 

Lengths. 

1  mil 

=  .025400 
=  .001 

millimeter 
inch 

TTfoTI 

a.404  8259 
,.0000000 

1  millimeter 

=  39.3708 

mils 

40      1.595  1741 

=  .039371 

inch 

Js     ,.595  1741 

1  centimeter 

=  .393708 

inch 

ft      T.595  1741 

1  inch 

=  2.53995 

centimeters 

V 

0.404  8259 

Ifoot 

=  .30479 

meter 

3 

T.484  0071 

1  yard 

=  .91438 

meter 

If 

,.961  1284 

1  meter 

=  39.370790 

inches 

40 

1.595  1741 

=  3.280899 

feet 

¥ 

0.515  9929 

" 

=  1.09363 

yards 

H 

0  038  8716 

1  kilometer 

=  3280.899 

feet 

3.515  9929 

** 

=  1093.633 

yards 

3.038  8716 

M 

=  .62138 

mile 

i 

,.793  3590 

1  mile 

=  5280. 

feet 

3.722  6339 

=  1760. 

yards 

3.245  5126 

M 

=  1609.31 

meters 

3.206  6410 

•• 

=  1.60931 

kilometers 

§ 

0.206  6410 

" 

=    .86838 

geog.  or  naut.  mile 

I 

T.938  7098 

1  geog.  or  naut.  mile 

=  6080.27 

feet 

3.783  9229 

Surfaces 

1  sq.  mil 

=  .00064514 

sq.  millimeter 

j.809  6518 

" 

=  .000001 

sq.  inch 

000  0000 

1  sq.  millimeter 

=  1550.1 

sq.  mils 

3^190  3482 

1  sq.  centimeter 
1  sq.  inch. 
1  sq.  decimeter 

=  .001550 
=  .15501 
=  6.4514 
=  15.501 

sq.  inch 
sq.  inch 
sq.  centimeters 
sq.  inches 

1 

-.190  3482 
T.190  8482 
0.809  6518 
1.190  3482 

u 

=  .10764 

sq.  foot 

X 

T.031  9858 

1  sq.  foot 

=  929.00 

sq.  centimeters 

2.968  0142 

" 

=  9.2900 

sq.  decimeters 

V 

0.968  0142 

1  sq.  yard 

=  .&3610 

sq.  meter 

T.922  2568 

1  sq.  meter 

=  10.764 

sq.  feet 

4.3. 

1.031  9858 

=  1.1960 

sq.  yards 

8 

0.077  7432 

1  are 
1  acre 

=  100. 
=  .40467 

sq.  meters 
hectare 

A 

2.000  0000 
T.607  1020 

" 

=  .0040467 

sq.  kilometer 

3.607  1020 

1  hectare 

=  10000. 

sq.  meters 

4.000  OtiOO 

4i 

=  2.4711 

acres 

B 

0.392  8980 

1  sq.  kilometer 

=  247.11 

acres 

2.392  8980 

1  sq.  mile 

=  .38612 
=  640. 

sq.  mile 
acres 

5g     T-586  7180 
2.806  1800 

" 

=  2.5899 

sq.  kilometers 

?g    10.413  2820 

270 


Equivalents  of  Units  of  Measurement. 


271 


Volun 

:ies. 

1  cub.  centimeter 

=  .061027 

cub.  inch 

A 

5.785  5223 

" 

=  .0-33816 

fluid  ounce 

3S 

5.529  1203 

44 

=  .0021135 

pint 

3.325  0003 

1  fluid  drachm 

=  3.6965 

cub.  centimeters 

V 

0.567  7897 

44 

=  .22559 

cub.  inch 

2 

T.353  3120 

1  cub.  inch 

=  16.386 

cub.  centimeters 

-325 

1.214  4777 

' 

=  .55411 

fluid  ounce 

* 

T.743  5980 

4 

=  .034632 

Eint 

2?tf 

5.539  4780 

4 

=  .016386 

ter 

ft 

5.214  4777 

1  fluid  ounce 

=  29.572 

cub.  centimeters 

30 

1.470  8797 

4 

=  8. 

fluid  drachms 

0.903  0900 

' 

=  1.8047 

cub.  inches 

§ 

0.256  4020 

1  pint 

=  473.15 

cub.  centimeters 

2.674  9997 

Si 

=  28.875 

cub  inches 

20Q 

1.460  5220 

44 

=  16. 

fluid  ounces 

1.204  1200 

44 

=  .47315 

liter 

u 

1.674  9997 

1  quart 

=  946.30 
=  57.7500 

cub.  centimeters 
cub.  inches 

2.976  0297 
1.761  5520 

" 

=  .94630 
=  .033420 

liter 
cub.  foot 

g 

T.976  0297 
5.524  0084 

1  liter 

=  1000. 

cub.  centimeters 

3.000  0000 

44 

=  61.027 

cub.  inches 

1.785  5223 

44 

=  2.1135 

pints 

« 

0.325  0003 

44 

=  1.0567 

quarts 

0.023  9703 

41 

=  .26419 

gallon 

K 

T.421  9103 

44 

=  .035317 

cub.  foot 

T7ff 

,.5479787 

1  gallon 

=  3785.2 

cub.  centimeters 

3.578  0897 

44 

=  231.0000 

cub.  inches 

zga 

2.363  6120 

44 

=  3.7852 

liters 

jyi 

0.578  0897 

44 

=  .13368 

cub.  foot 

A 

T.126  0683 

44 

=  .037852 

hectoliter 

A 

5.578  0897 

1  cub.  foot 

=  28815.3 

cub.  centimeters 

4.452  0213 

4 

=  29.922 

quarts 

30. 

1  .475  9916 

4 

=  28.3153 

liters 

1.452  0213 

4 

=  7.4805 

gallons 

If. 

0.873  9317 

4 

=  .28315 

hectoliter 

2 

T.452  0213 

4 

=  .028315 

cub.  meter 

A 

5-452  0213 

1  hectoliter 

=  105.67 

quarts 

2.023  9703 

4 

=  100. 

liters 

2.000  0000 

i 

=  26.419 
=  3.5317 

gallons 
cub.  feet 

1 

1.421  9108 
0.547  9787 

1  cub.^yard 

=  201.97 
=  .76451 

gallons 
cub.  meter. 

{3 

2.305  2955 
T.883  3852 

1  cub.  meter 

=  264.19 

gallons 

2.421  9103 

44 

=  35.317 

cub.  feet 

¥ 

1.547  9787 

44 

=  1.3080 

cub.  yards 

H 

0.116  6148 

1  stere 

=  1. 

cub.  meter 

0.000  0000 

272 


Equivalents  of  Units  of  Measurement, 


Weight. 

1  milligram 

=  .015432 

grain 

tin 

,.188  4320 

1  grain 

=  64.7'99 

milligrams 

1.811  5680 

1  gram 

=  15.43235 

grains 

15 

1.188  432  J 

44 

=  .035274 

ounce  avdp. 

3?U 

j.547  4539 

44 

=  .032151 

ounce  troy 

8 

5.  507  1908 

1  ounce  avdp. 

=  437.50 

grains 

2.640  97«1 

(4 

=  28.3495 
=  .91146 

grams 
ounce  troy 

11 

1.452  5461 
T.959  7369 

1  ounce  troy 

=  480. 

grains 

2.681  2412 

44 

=  31.1035 

grams 

31. 

1.492  8092 

1  pound  troy 

=  1.0971 
=  5760. 

ounces  avdp. 
grains 

ti 

0.040  2631 
3.760  4225 

44 

=  12. 

ounces  troy 

1.079  1812 

44 

=  .82286 

pound  avdp. 

§ 

T.915  3245 

41 

=  .37324 

kilogram 

i 

T.571  9905 

1  pound  avdp. 

=  7000. 

grains 

3.845  0980 

*c 

=  16. 

ounces  avdp. 

1.204  1200 

„ 

=  1.2153 
=  .45359 

pounds  troy 
kilogram 

if 

0.084  6755 
T.656  6660 

1  kilogram 

=  35.274 

ounces  avdp. 

754 

1.547  4539 

44 

=  2.2046 

pounds  avdp. 

H 

0.343  3340 

1  net  or  short  ton 

=  2000. 

pounds  avdp. 

3.301  0300 

44 

=  .90719 

metric  ton 

i? 

T.957  6960 

44 

=  .89286 

long  ton 

ft 

T.950  7820 

1  metric  ton 

=  2204.62 

pounds  avdp. 

3.343  3340 

44 

=  1.1023 

short  tons 

H 

0.042  3040 

44 

=  .98421 

long  ton 

H 

T.993  0860 

1  gross  or  long  ton 

=  2240. 

pounds  avdp. 

3.350  2480 

fc* 

=  1.1200 

short  tons 

y 

0.049  2180 

" 

=  1.01605 

metric  tons 

ft 

0.006  9140 

Weights  and 

Lengths. 

1  Ib.  per  mile 

=  .28185 

klgr.  per  kilometer 

? 

T.450  0250 

44 

=  .11048 

grain  per  inch 

I 

r.043  2829 

44 

=  .0028185 

grain  per  centimeter 

vil 

,.450  0250 

1  klgr.  per  kilomet. 

=  3.5479 

Ibs.  per  mile 

£     5.5499750 

41 

=  .39197 

grain  per  inch 

TO      T.593  2579 

44 

=  .01000 

gram  per  centimeter 

3.000  0000 

1  grain  per  inch 

=  9.0514 

Ibs.  per  mile 

9. 

0.956  7171 

u 

M 
U 

=  2.5512 
=  .02.5512 
=  .00255-12 
=  .0017143 

klgs.  per  kilometer 
gram  per  centimeter 
klg.  per  meter 
Ib.  per  foot 

i 

& 

0.406  7421 
5.406  7421 
3.406  7421 
-.234  0832 

1  gram  per  centimet. 

=  354.79 
=  100.000 

Ibs.  per  mile 
klgs.  per  kilometer 

70Q 

2.549  9750 
2.000  0000 

44 

=  39.197 

grains  per  inch 

40 

1.593  2579 

K 

=  .10000 

klg.  per  meter 

T.OOO  0000 

44 

-  .067196 

Ib.  per  foot 

A 

,.827,3411 

1  klg.  per  meter 

=  391.97 
=  10. 

grains  per  inch 
grams  per  centimeter 

400 

2.593  2579 
1.000  OMXJ 

i* 

1  Ib.  p^er  foot 

=  .67196 
=  583.33 

=  14.882 

Ib.  per  foot 
grains  per  inch 
grams  per  centimeter 

1 

!.827  3411 
2.765  9168 
1.1726589 

H 

=  1.4882 

klgs.  per  meter 

* 

0.172  658G 

Equivalents  of  Units  of  Measurement. 


273 


Weights  and  Surfaces  (Pressures). 

1  Ib.  per  sq.  inch            =.068044         atmosphere 

ft 

5.832  7894 

=  .070310         klgr.  per  sq.  centimeter 

7 
"1  7JTJ 

5.847  0142 

1  klgr.  per  sq.  centim.  =14.223           Ibs.  per  sq.  inch 

ill 

1.152  9858 

=  .  96778          atmospl  i  ere 

i? 

r  985  7752 

1  atmosphere                 =760.              millimeters  of  mercury  col. 
=  33  .  901           feet  water  column 

34 

2  '.880  8136 
1.530  2177 

=  29.922           inches  of  mercury  column 

30. 

1.475  9877 

=  14.(>%           Ibs.  per  sq.  inch 

^^ 

1.167  2106 

=  10.333           meters  water  column 

31 

1.014  2248 

=  1  .0333           klgs.  per  sq.  centimeter 

31 

0  014  2248 

sp.  gr.  of  mercury         =  13.596 

9 

T.  133  4112 

Weights  and  Volumes. 

1  grain  per  cub.  inch     =  .24686           Ib.  per  cub.  foot 

i 

T.392  4457 

=  .0039545        gram  per  cub.  centimeter 
1  Ib.  per  cub.  foot          =  4.0509           grains  per  cub.  inch 

I04Z5<J 

4 

3.597  0903 
0.607  5543 

=  .016019         gram  per  cub.  centimeter 

*§* 

5.204  6447 

=  .016019         kilgr.  per  liter 
1  gram  per  cub.  c.  m.    =  252.88           grains  per  cub.  inch. 
1  gram  per  cub.  c.  m.    =  62.425           Ibs.  per  cub.  foot 

IS. 

BOP 

5.204  6447 
2.402  1097 
1.795  3553 

1  klgr.  per  liter               =  62.425           Ibs.  per  cub.  foot 

1.795  3553 

=  2.0862           Ibs  per  quart 

0.319  3637 

lib.  per  quart                 =.47933           klgr.  per  liter 

j.680  6363 

1  Ib.  per  cub.  inch           =  .027681         klgr.  per  cub.  c.  m. 

5.442  1883 

1  klgr.  per  cub.  c.  m.      =  36.125           Ibs.  per  cub.  inch 

36. 

1.557  8117 

Weight  of  Water.    K. 

1  cub.  c.  m.  water  weighs  15.432           grains 

15. 

1.188  4320 

1  .                  gram 

1 

0.000  0000 

1  cub.  inch  weighs              252.88           grains 

2.402  9097 

16.386           grams 

1.214  4777 

.036125         pound 

5.557  8117 

1  quart  weighs                    2.0862           pounds 

0.319  3637 

.94630           kilogram 
1  litre  weighs                       2.2046           pounds 

1 

1.976  0297 
0.343  3840 

1  cub.  foot  weighs              62.425           pounds 

1.795  3553 

28.3153         kilograms 

ITO 

1.452  0213 

1  cub.  yard  weighs             1685.5           pounds 

3.226  7192 

764.51           kilograms 

2.8P3  3852 

1  cub.  metre  weighs          2204.6          pounds 

3.343  3340 

M. 

1  pound  water  measures  453.  59           cub.  centimeters 

2  656  6660 

27.681           cub.  inches 

_B5 

1  442  1883 

"                        .47933           quarts 

If 

T.680  6363 

.45359           liter 

?o 

j.656  6600 

.016019         cub.  foot 

81 
5 

5  204  6447 

1  klgr.  water  measures     61  .027          cub.  inches 

1.785  5223 

1.0567           quarts 

15 

0.023  970.5 

"                          1.                  liter 

0.000  0000 

"                           .035317         cub.  foot 

rf* 

5.547  9787 

The  weight  of  a  given  volume  of  any  other  material,  whether 
solid  or  liquid,  is  its  specific  gravity  multiplied  by  the  factor  K. 
The  volume  of  a  given  weight  of  any  other  material,  is  the  fac- 

tor M  divided  by  its  specific  gravity. 

274 


Equivalents  of  Units  of  Measurement. 


Velocities. 

1  foot  per  second 

=  .30479           meter  per  second 
=  .018288         klmet.  per  minute 

* 

T.484  0071 
-.262  1583 

1  meter  per  second 

=  .011364         mile  per  minute 
=  3.280899        feet  per  second 
=  .0600            klmet.  per  minute 

l     > 

vr.055  5173 
0.515  9929 
j.778  1513 

=  .037283         mile  per  minute 
1  kilometer  per  minute  =  54.682           feet  per  second 
=  16.667           meters  per  second 

5  . 

1     0 

».ft71  5102 
1.737  8417 
1.221   84S? 

" 

=  .62138           mile  per  minute 

1.793  3~){K) 

1  mile  per  minute 

=  88.00             feet  per  second 
=  26.822            meters  i  >er  second 

27 

1.944  4827 
1.428  489H 

" 

=  1.60931          klmet.  per  minute 

1 

0.206  6110 

Gravity. 

ArPPiPrntinn  nf  Ov              /  981-000      centimeters  per  second 
leration  of  gravity  =  j  32  186      feet  per  second 

1000. 

H* 

2.991  6690 
1.507  6619 

Forces  (see  also  Weights). 

1  dyne 

=  1.0194           milligrams 

i. 

0.008  3310 

" 

=  .015731          grain 

5B5 

5.196  7630 

" 

=  .0010194        gram 

i 

nvv 

3.008  3310 

« 

=  .00003596      ounce  avoirdupois 

5.555  7849 

1  milligram 
1  grain 
Igram 

=  .981               dyne 
—  63.568           dynes 
-  981. 

1. 

A.12 

1000 

T.991  6690 
1.803  2370 
2.991  6690 

1  ounce  avdp. 

=  27811. 

4.444  2151 

1  pound  avdp. 
1  kilogram 

=  444976.               " 
=  981000. 

5.648  3351 
5.991  6690 

Work. 

lerg 

=  1.                 dyne-centimeter 

1 

0.000  0000 

11 

=  .0000001        joule 

7.000  oooo 

1  gram-centimeter 

=  981.00           ergs 

1000 

2.991  6690 

1  foot-grain 

=  .00001           kilogram-meter 
=  1937.5           ergs 

y.OOO  0000 
3.287  2441 

1  joule,  or 

=  10,000,000. 

7.000  0000 

1  volt-coulomb,  or 

=  .737324          foot-pound 

J 

T.867  6580 

1  watt  during  every 
second,  or 
1  volt-ampere  during 

=  .101937        kilogram-meter 
=  .0013592        metric  horse-power  for  one 
second 

1 

T.008  3310 
3.133  2698 

every  second 

=  .0013406       horse-power  for  one  second 

TIT5%CT 

3.127  2954 

" 

=  .0009551        pound-Fall.,  heat  unit 

j.980  0407 

" 

=  .0005306       pound-Centig.,  heat  unit 

TT§tJ5 

x.724  7682 

" 

=  .0002407        kilogr.-Ccntig., 

12 

3.381  '4342 

M 

=  .0002778        watt-hour 

a 

3.443  6975 

1  foot-pound 

=  13562600.       ergs 

. 

7.132  3420 

<i 

=  1.35626         joules 

4 

0.132  3420 

" 

=  .13825          kilogram-meter 

1 

T.140  6730 

H 

=  .0018434        metric  horse-power  for  one 

second 

TT<TU 

3.265  6117 

" 

=  .00181818      horse-power  for  one  second 

,.259  6373 

Equivalents  of  Units  of  Measurement. 


275 


Work.—  (Continued.) 

f 

1  foot-pound 

=  .0012953 

pound-Fah.  ,  heat  unit 

sifoTj 

3.112  3827 

,',' 

=  .0007196 
=  .0003264 

pound-Centig.,  heat  unit 
kilogr.-Centig., 

7cW 

3~510"<J 

¥.857  1102 
,.513  7762 

" 

=  .0003707 

watt-hour 

j.576  0395 

1  kilogram-meter 

=  98100000. 

ergs 

7.991  6690 

=  9.81000 

joules 

10. 

0.991  6690 

=  7.23314 

foot-pounds 

¥ 

0.859  3270 

=  .01333 

metric  horse-power  for  one 

second 

SSO- 

5.124  9387 

=  .013151 

horse-power  for  one  second 

300 

Uuo* 

5.118  9643 

=  .009369 

pound-Fah.,  heat  unit 

•]• 

-.971  7097 

=  .005205 

pound-Centig.,  heat  unit 

rife 

3.716  4372 

=  .002:  '.61 

kilogr.-Centig., 

sftv 

3.373  1032 

=  .00272f> 

watt-hour 

T&iJ 

3.435  3670 

1  watt-hour 

=  3600. 

joules 

3.556  3025 

=  2654.4 

foot-pounds 

uyia 

3.423  9605 

=  366.97 

kilogram-meters 

2.564  6335 

=  3.4383 

pound-Fah.,  heat  units 

0.536  3433 

=  1.9102 

==  .8664 

pound-Centig.,  heat  units 
kilogr.-Centig.,        " 

0.281  0708 
T.937  7368 

=  .0013592 

metric  horse-power-hour 

3.133  2698 

' 

=  .0013406 

horse-power-hour 

30%ff 

3.127  2954 

1  metric  horse-po\v 

er- 

hour 

=  2648700. 

joules 

6.423  0327 

=  1952940. 

foot-pounds 

6.290  6908 

=  270000. 

kilogram-meters 

5.431  3638 

=  2529.7 

pound-Fah.,  heat  units 

JLQOfiQ 

3.403  0735 

=  1405.4 

pound-Centig.  ,  heat  units 

IJLQQ 

3.147  8010 

=  637.5 

kilogr.-Centig.,        " 

600fi 

2.804  4670 

=  735.75 

watt-hours 

3i)00 

2.866  7302 

1  horse-power-hour 

=  .98634 
=  2685400. 
=  1980000. 

horse-power-hour 
joules 
foot-pounds 

1? 

T.994  0256 
6.429  0071 
6.296  6652 

=  273740. 

kilogram-meters 

5.437  3382 

=  2564.8 
=  1424.9 

pound-Fah.,  heat  units 
pound-Centig.,  heat  units 

10000 
JOOOO 

3.409  0479 
3.153  7754 

=  646.31 

kilogr.-Centig., 

2000 

2.810  4414 

=  745.941 

watt-hours 

SfiLftO 

2.872  7046 

K 

=  1.01385 

metric  horse-power-hours 

« 

0.005  9744 

Heat. 

1  gram-Centigrade 
1  pound-Fahrenheit 

-.001 
=  1047.03 

=  772. 

k  ilogram-Centigrade 
joules 
foot-pounds 

T«Vn 

^IQO 
3.0J12 

3.000  0000 
3.019  9593 
2.887  6173 

=  106.731 
=  .55556 
=  .25200 

kilogram-meters 
pound-Centigrade 
kilogram-Centigrade 

3|Q 

2.028  2903 
T.744  7275 
T.401  3935 

=  .29084 

watt-hour 

2 

T.463  6567 

=  .0003953 

metric  horse-power-hour 

Tffo^TI 

1UUUU 

?.596  9265 

=  .0003899 

horse-power-hour 

TTTOTT7T 

,.590  9521 

1  pound-Centigrade 

=  1884.66 

=  1389.6 

joules 
foot-pounds 

Iiooo 
70QO 

3.275  2318 
3.142  8898 

=  192.116 
=  1.8000 

kilogram-meters 
pound-Fahrenheit 

f 

2.283  5628 
0.255  2725 

276 


Equivalents  of  Units  of  Measurement. 


Heat  —  (Continued.) 

1  pound-Centigrade 

=  .4536 

kilogram-Centigrade 

A 

T.656  6660 

* 

=  .52352 

watt-hour 

il 

T.718  9292 

1  kilogram  Centigrade 

=  .0007115 
=  .0007018 
=  4154.95 

metric  horse-power-hour 
horse-power-hour 

joules 

5 

700  5 
TooTiTT 

GOOOO 

.f..sr)2  It  »90 
3!  618  5658 

* 

-  3063.5 

foot-pounds 

3000. 

3.486  2238 

i 

=  423.54 

kilogram-meters 

soao 

2.626  8968 

' 

=  3.9683 

pound-Fahrenheit 

I 

.598  6065 

« 

=  2.2046 

pound-Centigrade 

0.343  3340 

< 

=  1.1542 

watt-hours 

0.062  2632 

1 

=  .001569 

metric  horse-powrer-hour 

3.195  5330 

=  .0015472 

horse-power-hour 

2THJ5 

,.189  5586 

Power. 

1  erg  per  second 

=  .0000001 

watt 

-.000  0000 

1  watt,  or 

=  10000000. 

ergs  per  second 

7.000  0000 

1  volt-ampere,  or 
1  joule  per  second,  or 

=  44.2394 
-  6.11622 

foot-pounds  per  min. 
kilogram-meters  per  min. 

1 

1.645  8093 
0.786  4823 

1    volt-coulomb    per 
second 

=  .0573048 
-  .0318360 

Ib.-Fah.,  heat  unit  per  min. 
Ib.-Cent, 

& 

,.758  1920 
5.502  9195 

" 

=  .0144402 

klgr.-Cent., 

*1_ 

,.159  5855 

" 

-  .0013592 

metric  horse-power 

3THT5 

-.133  2698 

1  foot-pound  per  min. 

=  .0013406 

=  226043. 

horse-power 
ergs  per  second 

nw* 

7.127  2954 
5.354  1907 

** 

=  .0226043 

watt 

A 

7T.:'..">4  1907 

« 

=  .13825 
=  .00003072 
=  .000030303 

kilogram-meter  per  min. 
metric  horse-power 
horse-power 

.00003 

.00003 

T.140  6730 
-.487  4605 
P.  481  4861 

1    kilogram-meter   per 

minute 

=  1635000. 

ergs  per  second 

6.213  5177 

** 

=  .163500 

watt 

i 

T.213  5177 

\\ 

=  7.23314 
=  .0002222 

foot-pounds  per  minute 
metric  horse-p<n\rr 

).xf>9  3270 
1.346  7874 

1  metric  horse-power,  ' 

=  .0002192 
=  735.75  xlO7 

horse-power 
ergs  per  second 

rare 

,.340  8130 
9.866  7302 

or 

=  735.750 

watts 

son.o 

2.S66  73(12 

1  French  horse-power, 

=  32549.0 

foot-pounds  per  minute 

Ifloooo 

4.512  5395 

or 
1  cheval-vapeur,  or 
1  force  de  cheval,  or 

-4500. 
=  42.162 
=  23.423 

kilogram-meters  per  min. 
Ib.-Fah.,  heat  units  per  min. 
Ib.-Cent., 

3.653  2125 
1.624  9222 
1.369  '6497 

1  Fferd  kraft 

=  10.625 

klg.-Cent., 

1.026  3157 

' 

=  .98634 

horse-power 

T.994  0256 

1  horse  power 

=  745.94  xlO7 

ergs  per  second 

J.S72  T046 

=  745.941 

watts 

3000 

2.872  7046 

. 

=  33000. 
-  4562.33 

foot-pounds  per  minute 
kilogram-meters  per  min. 

1Q{M)J)Q- 
900_Q 

4.518  5139 
3.659  1869 

=  42.746 

Ib.-Fah.,  heat  units  per  min. 

soa 

1.630  8960 

-  23.748 

Ib.-Cent., 

7_Q 

1.375  6241 

=  10.772 

klg.-Cent., 

11. 

1.032  2901 

=  1.01385 

metric  horse-powers 

fi     0.005  9744 

Equivalents  of  Units  of  Measurement. 


277 


Power.—  (Continued.) 

1  Ib.-Fah.,  heat  unit  per 

min.                            =  17.45xl07     ergs  per  second 

8.241  8080 

=  17.4505          watts 

7-Q 

1.241  8080 

=  .023718         metric  horse-power 

SOtJ 

5.375  0778 

=  .023394          horse-power 

"soiy 

5.369  1034 

1   Ib.-Cent,,  heat  unit 

per  min.                       =31.41x10*     ergs  per  second 

8.497  aS05 

=  31  .4109          watts 

10O0 

1.497  0805 

=  .04269           metric  horse-power 

7J 

5.630  3503 

=  .042109         horse-power 

5.624  3759 

1  klgr.-Cent,,  heat  unit 

per  min.                      =  69.25xl07     ergs  per  second 

8.840  4145 

=  69.24!)            watts 

70. 

1.840  4145 

=  .09412           metric  horse-power 
=  .092835         horse-power 

s 

5.973  6843 
5.967  7099 

Circular  (Cross-section)  Units.* 

1  circular  mil                 =  .78540           square  mil 
=  .00064514      circular  millimeter 

5'fiW 

T.895  0899 
,.809  6518 

=  .00050669      square  millimeter 

j.704  7417 

1  square  mil                  =1.2732          circular  mils 

'jU) 

0.104  9101 

=  .00082141      circular  millimeter 

1 

x.914  5619 

1  circular  millimeter     =  1550.1           circular  mils 

afijlfl 

3.190  3482 

=  1217.4            square  mils 

eooQ 

3.085  4381 

=  .78540           square  millimeter 
1  square  millimeter       =  1973.6           circular  mils 

2$b. 

T.895  0899 
3.295  2583 

=  1.2732           circular  millimeters 

V 

0.104  9101 

If  <l  is  the  diameter  of  a 

circle,  the  area  in  other 

units  is  : 

If  d  is  in  mils,  area  in 

sq.  millimeters            =  d~x.  00050669 

^nrffff 

5.704  7417 

d  in  millimeters,  area  in 

sq.  mils                         =  d2x!217.4 

0<M)Q 

3.085  4381 

d  in  centimeters,  area 

in  sq.  inches                =  d2x.  12174 

6 

T.085  4381 

d.  in  inches,  area  in  sq. 

centimeters                  =  cZ2x5.0669 

5 

0.704  7417 

*  A  circular  unit  is  the  area  of  a  circle  whose  diameter  is  one 

unit, 

Electrical  Resistance. 

1  Siemens  or  mercury 

unit                               =  .9540             B.  A.  unit 

if 

T.979  5312 

=  .9134             legal  ohm 

§f 

T.974  6941 

1  B.  A.  unit                     =  1.0483           Siemens  units 

0.020  4688 

=  .9889             legal  ohm 

j 

T.995  1629 

1  legal  ohm                     =  1  .0600           Siemens  units 

14 

0.025  3059 

=  1.0112            B.  A.  units 

i: 

0.004  8371 

278 


Equivalents  of  Units  of  Measurement. 


Specific  Electrical  Resistance. 

1  ohm  per  foot  per  circ. 
mil  or  per  mil  diam.  =  .78540           ohm  per  foot  per  sq.  mil 
=  .16624           microhm   per  cubic  centi- 

ft 

T.895  0899 

meter 

i 

r.220  7346 

=  .0021166        ohm  per  meter  per  m.  m. 

diam. 

__21 

3.325  6447 

=  .0016624        ohm  per  meter  per  sq.  m.  m. 

BO(J 

3.220  7346 

1  ohm  per  foot  per  sq. 

mil                                =  1.2732           ohms  per  foot  per  mil  diam. 
—  .21166           microhm  per  cubic   centi- 

V 

0.104  9101 

meter 

J!lff 

T.325  6447 

=  .0026950        ohm  per  meter  per  m.  m. 

diam. 

noffti 

3.430  5548 

=  .0021166       ohm  per  meter  per  sq.  m.  m. 

TcToTJIT 

g.325  6447 

1   microhm  per   cubic 

centimeter                  =  6.0154           ohms  per  foot  per  mil  diam. 
=  4.7245           ohms  per  foot  per  sq.  mil 

6 

0.779  2654 
0.674  3553 

=  .012732         ohm  per  meter  per  m.  m. 

diam. 

& 

^.104  9101 

=  .01                ohm  per  meter  per  sq.  m.  m. 
1  ohm  per  meter  per  cir- 
cular millimeter  or  per 

.01 

5.000  0000 

millimeter  diam.         =  472.45           ohms  per  foot  per  mil  diam. 

U^OQQ 

2.674  3553 

=  371.06           ohms  per  foot  per  sq.  mil 
=  78.540           microhms  per  cubic  centi- 

2.569 4452 

meter 

80 

1.895  0899 

=  .78540           ohm  per  meter  per  sq.  m.  m. 
1  ohm  per  meter  per  sq. 

15 

1.895  0899 

millimeter                   =  601.54           ohms  per  foot  per  mil  diam. 
=  472.45           ohms  per  foot  per  sq.  mil 

600. 
uypa 

2.779  2654 
2.G74  3553 

=  100.              microhms  per  cubic  centi- 

meter 

100. 

2.000  0000 

=  1.2732           ohms  per  meter  per  m.  m. 

diam. 

V 

0.104  9101 

Electrical  Quantity. 

1  coulomb                      =  .0002778        ampere-hour 

TTOOTJ 

3.443  6975 

1  ampere-hour               =  3600.             coulombs 

3.556  3025 

Absolute  Electrical  Units. 

1  coulomb                      =  10"1  electromagnetic  units  of  quantity 

1  ampere                        =  10"1               "                    "      current 

1  volt                             =  108                "                    "      e.  m.  f. 

1  ohm                             =  109                "                    "      resistance 

1  farad                            =  10~9               "                    "      capacity 

1  joule                            =  107   absolute  units  of  work  (ergs) 

1  watt                             =  107   absolute  units  of  power  (ergs  per 

second) 

* 

1  micro  =  1  millionth 

1  mega  =  1  million 

1  foot-pound                I  =  f 

1  kilogram-meter         1=1  See  ergs  and  ergs  per  second  in  tables 
1  metric  horse-power  |  =  1     of  Work  and  Power. 

1  horse-power 

Equivalents  of  Units  of  Measurement.  279 

"  Dimensions  "  of  Mechanical,  Electrical  and  Magnetic  Units. 
m = mass,  I — length,  t  =  time. 

Surface I* 

Volume  (capacity) ? 

Velocity It'1 

Acceleration It  * 

Force mlt  * 

Weight mlf* 

Work  or  heat  (joule,  foot-lbs.  or  heat  units) mfC* 

Power  (rate  of  work,  watt,  horse-power) m  I  t  3 

I  \ 
Electrical  quantity  * ml 

Current* mlt'1 

Electromotive  force  * m  I  T* 

Resistance  * If" 

Capacity  * I  1 1 

Magnetic  quantity  (magnetic  lines  of  force) mlt 

Magnetic  intensity ml    t~l 

*In  the  electromagnetic  system. 


464518 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


